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PubMed2025

Pentatricopeptide repeat protein targeting CUG repeat RNA ameliorates RNA toxicity in a myotonic dystrophy type 1 mouse model.

Myotonic dystrophy type 1 (DM1) is an autosomal dominant multisystemic disorder caused by the expansion of a CTG-triplet repeat in the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene. It results in the transcription of toxic RNAs that contain expanded CUG repeats (CUGexp). Splicing factors, such as muscleblind-like 1 (MBNL1), are sequestered by CUGexp, thereby disrupting the normal splicing program that is essential for various cellular functions. Pentatricopeptide repeat (PPR) proteins, originally found in plants, regulate RNA in organelles by binding in a sequence-specific manner. Here, we designed PPR proteins that specifically bind to the hexamer of CUG repeat RNAs (CUG-PPRs) and showed that CUG-PPR1 could ameliorate RNA toxicity induced by CUGexp in cell models of DM1. A single systemic recombinant adeno-associated virus (AAV9) vector-mediated gene delivery of CUG-PPR1 demonstrated long-term therapeutic effects on myotonia and restored splicing activity in a mouse model of DM1. These results highlight the potential of PPR molecules to target pathogenic RNA sequences in DM1 and potentially other RNA-mediated disorders.

Animals↗

PubMed1992

Characterization of a herpes simplex virus sequence which binds a cellular protein as either a single-stranded or double-stranded DNA or RNA.

Earlier we reported that herpes simplex virus 1 DNA contains a sequence which binds a host protein in a sequence-specific manner as either a single-stranded or a double-stranded DNA or RNA and that this sequence is located in a transcriptional unit whose RNA traverses the origin of viral DNA replication (OriSRNA) (R.J. Roller, L. McCormick, and B. Roizman, Proc. Natl. Acad. Sci. USA 86:6518-6522, 1989). The protein reacts with both DNA and RNA in band-shift assays and protects the single-stranded RNA sequence from digestion by RNase. We report that the minimal cognate sequence required for these interactions consisted of [N(GTGGGTGGG)2(N less than or equal to 10)]. The ninemer repeat sequence was located at nucleotides -1 to -18 relative to the transcription initiation of the major species of OriSRNA. The activity of the cognate sequence required at least three guanines between thymines and tolerates the insertion of additional thymines, but it was inactivated by the insertion of adenines or by the substitution of some of the guanines with cytosines in one repeat. Replacement of the 10 3' nucleotides has no effect on binding activity, whereas deletion of these sequences abolished it. Among the related sequences with no demonstrable binding activity were some telomeric sequences which interact with known cognate proteins. The electrophoretic mobility of the herpes simplex virus cognate sequences in nondenaturing gels suggests that they may be able to form higher-order structures, but the conditions under which they were formed were different from the optimal conditions for binding the protein. UV light cross-linking studies of labeled RNA-protein complexes following digestion with RNases indicated that the electrophoretic mobility of the protective activity corresponded to that of a protein with an apparent molecular weight of 100,000.

Base Sequence↗

PubMed1992

The herpes simplex virus 1 RNA binding protein US11 is a virion component and associates with ribosomal 60S subunits.

The herpes simplex virus 1 US11 gene encodes a site- and conformation-specific RNA binding regulatory protein. We fused the coding sequence of this protein with that of beta-galactosidase, expressed the chimeric gene in Escherichia coli, and purified a fusion protein which binds RNA in the same way as the infected cell protein. The fusion protein was used to generate anti-US11 monoclonal antibody. Studies with this antibody showed that US11 protein is a viral structural protein estimated to be present in 600 to 1,000 copies per virion. The great majority of cytoplasmic US11 protein was found in association with the 60S subunit of infected cell ribosomes. US11 protein associates with ribosomes both late in infection at the time of its synthesis and at the time of infection after its introduction into the cytoplasm by the virion. US11 protein expressed in an uninfected cell line stably transfected with the US11 gene associates with ribosomal 60S subunits and localizes to nucleoli, suggesting that US11 protein requires no other viral functions for these associations.

Antibodies, Monoclonal↗

PubMed1992

High density lipoprotein-binding proteins in porcine liver. Isolation and histological localization.

The antiatherogenic properties of high density lipoproteins (HDLs) are thought to reside in their involvement in the reverse cholesterol transport pathway. Specific HDL-binding proteins could play a key role in this process. Two HDL-binding proteins of approximately 90 and 180 kd were identified in porcine liver by ligand blotting and were purified to apparent homogeneity by a combination of protein extraction, DEAE-cellulose chromatography, Con A-Sepharose chromatography, and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Binding of 125I-HDL by these proteins could be actively competed for by unlabeled HDL but not by low density lipoprotein. Polyclonal antisera have been raised against these two proteins. Each antiserum recognized only one of the HDL-binding proteins, indicating that they are not immunologically related. Moreover, striking differences in localization were observed in immunohistochemical studies. The 90-kd protein is located within the hepatocellular plates, while the 180-kd protein is present along the lining of the sinusoids. These results suggest functional differences between the two HDL-binding proteins described.

Animals↗

PubMed2026

Tracing the molecular route to progression in miRNA-biogenesis-defective thyroid lesions.

Germline and somatic changes in DICER1 and DGCR8 microprocessors confer risk of developing benign and malignant thyroid lesions, yet the molecular events driving malignant transformation remain unclear. We trace the molecular trajectories from benignity to malignancy in DICER1- and DGCR8-mutated thyroid lesions using multiomic profiling on over 30 DICER1-/DGCR8-mutated samples. Our findings reveal a progressive, specific, and linear accumulation of genetic changes, which when combined with enhanced downregulation of miRNAs distinguished DICER1-/DGCR8-malignant lesions from their benign counterparts. Compensatory hypomethylation of miRNA-encoding genes characterized DICER1-/DGCR8-benign lesions, but as the tumors progressed to malignancy, methylation was partly reimposed, reversing the attempts to activate miRNA-encoded genes and further compromising miRNA production. Transcriptomic analyses revealed mutation-specific effects on the microenvironment, whereby DICER1 mutations activated canonical thyroid cancer progression pathways, whereas altered DGCR8 associated with immune-related changes. This work unveils specific molecular events underlying malignant progression of miRNA-biogenesis-related thyroid tumors and identifies potential biomarkers and disease etiology mechanisms.

MicroRNAs↗

PubMed2026

Identification of a necroptosis-related lncRNA prognostic signature and the hub RBP HNRNPK in esophageal squamous cell carcinoma.

ObjectiveEsophageal squamous cell carcinoma (ESCC) is a malignant tumor with poor prognosis. Necroptosis is important for tumor immunity, but its role in ESCC remains unclear. This retrospective bioinformatics study aimed to investigate the prognostic value of necroptosis-related long non-coding RNAs (lncRNAs) and to identify key lncRNA-binding proteins (RBPs) in ESCC patients.MethodsRNA transcriptome and clinical data of ESCC patients were obtained from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) databases. Necroptosis-related lncRNAs were identified through correlation analysis with necroptosis-related genes, subjected to consensus cluster analysis, and used to construct a prognostic risk model via least absolute shrinkage and selection operator (LASSO) regression. The hub RBP was experimentally validated by quantitative polymerase chain reaction (qPCR) using 30 pairs of ESCC and adjacent normal tissues from patients who underwent surgical resection.ResultsA total of 30 necroptosis-related lncRNAs were significantly correlated with overall survival (OS). The upregulated lncRNAs in the risk model were associated with high immune scores, innate immune cell infiltration, cluster 2 classification, and advanced T-stage disease (p < 0.05). Three hub RBPs (HNRNPA1, HNRNPC, and HNRNPK) were identified through protein-protein interaction network analysis. qPCR confirmed that HNRNPK was significantly overexpressed in ESCC tissues compared to adjacent normal tissues (p < 0.05).ConclusionsThe necroptosis-related lncRNA risk model is an independent prognostic factor for ESCC patients. HNRNPK was identified as a hub RBP significantly overexpressed in ESCC tissues. We hypothesize that HNRNPK may promote tumor progression through regulating proto-oncogene expression or modulating the immune microenvironment, though this requires further mechanistic validation.

Humans↗

PubMed2026

The N6-methyladenosine reader IGF2BP2 in T-cell lymphoma.

Peripheral T-cell lymphoma (PTCL) represents a highly heterogeneous and aggressive lymphoid neoplasm that lacks pathogenic biomarkers of RNA modification with therapeutic potential. IGF2BP2 is recognized as an N6-methyladenosine reader critically involved in oncogenesis. In this study, we observed consistently high expression of IGF2BP2 across common nodal PTCL subtypes in 3 independent external cohorts, which was further confirmed in our RNA-sequencing (RNA-seq) data set of 196 patients with newly diagnosed PTCL. Both in vitro and in vivo, IGF2BP2 promoted tumor cell growth and inhibited CD8+ T-cell infiltration within the tumor microenvironment. Mechanistically, IGF2BP2 bound to endosome-related genes (RAB4, VPS35, RAB9, and STAM) to maintain their stability, which resulted in enhanced endocytic activity and increased internalization of membrane proteins, and ultimately induced tumor cell proliferation and inhibition of CD8+ T-cell-mediated tumor cytotoxicity. The relationship between IGF2BP2 and endocytosis-associated genes was confirmed using RNA-seq data from patients with PTCL. IGF2BP2 as an upstream regulator of both tumor growth and immune suppression was further demonstrated in patient-derived xenograft models and a coculture system established using tumor samples from patients with PTCL and peripheral blood mononuclear cells. Notably, therapeutic targeting of IGF2BP2 with CWI1-2 suppressed endocytosis and impeded tumor growth in both cell lines and patient-derived xenograft models. Collectively, our findings highlight IGF2BP2 as a clinically relevant oncogenic driver in PTCL that integrates tumor-intrinsic growth signals with immune evasion through endocytosis-centered regulation, providing a novel therapeutic rationale for RNA modification-based strategies that concurrently target tumor cells and the tumor microenvironment.

Humans↗

PubMed2026

Genetic variants in IGF2BP family genes are associated with glioma risk in Chinese children.

BACKGROUND: Glioma is a highly prevalent malignant tumor of the central nervous system in children and is driven by complex genetic and environmental factors. IGF2BP family genes (IGF2BP1, IGF2BP2, and IGF2BP3) encode critical RNA epigenetic "readers" that participate in posttranscriptional gene regulation and modulate various cellular processes. However, the contributions of these gene variants to glioma risk remain unclear. METHODS: A multicenter case-control study was conducted, enrolling 360 patients with glioma and 547 cancer-free controls. Genotyping of 11 potentially functional polymorphisms within IGF2BP family genes was performed using the TaqMan assay. Unconditional logistic regression models were employed to estimate odds ratios and 95% confidence intervals. Furthermore, expression quantitative trait loci (eQTL) and The Cancer Genome Atlas (TCGA) clinical database analyses were conducted to investigate the potential regulatory mechanisms and clinical significance of the identified variants. RESULTS: We found that the IGF2BP1 rs2270575 polymorphism was significantly associated with a decreased risk of glioma. Conversely, the IGF2BP2 rs17289925 and rs7646419 polymorphisms were linked to increased glioma risk. Stratification and cumulative effect analyses revealed that harboring multiple risk genotypes of IGF2BP1 or IGF2BP2 substantially increased glioma susceptibility. This cumulative risk was especially notable among males, younger children (< 60 months), and patients diagnosed with early-stage (I + II) tumors. Additionally, eQTL and TCGA analyses revealed that the rs2270575 and rs7646419 alleles correlated significantly with altered mRNA expression levels of CALCOCO2 and AC099661.1 (ENSG00000286086), respectively, which further correlated with favorable molecular subtypes (IDH mutation status) and WHO tumor grades. CONCLUSION: Genetic polymorphisms within IGF2BP family genes significantly modulate pediatric glioma susceptibility and have cumulative, subtype-specific, and age-dependent effects; thus, these genes may serve as promising noninvasive biomarkers for early risk stratification of childhood glioma.

Adolescent↗

PubMed2026

Riboregulation: a non-canonical tau function.

Almost since its discovery, tau protein has perplexed scientists and clinicians with its varied roles in physiology as well as its appearance as phosphorylated protein aggregates of various structures in many neurodegenerative diseases. Tau plays a role in microtubule stabilization, but from the earliest of studies, tau has also been observed to bind to RNA, with recent research suggesting tau has a higher affinity for some RNA species compared to microtubules. In the context of disease, tau dysfunction potentiates disruptions to RNA metabolism, including the perturbation of mRNA splicing, impairment of translation, de-repression of transposable elements, and alteration of RNA export and degradation. Tau aggregates directly sequester diverse RNA species and RNA binding proteins. Emerging evidence reinforces the characterization of tau as an RNA binding protein, highlighting questions about both the physiological and disease-related functions of this direct RNA binding. The disparate structure of tau in normal and various disease states makes teasing apart the various impacts on RNA and regulation a more difficult puzzle requiring future study. In this review, we summarize the evidence for tau's role in RNA biology, including as an RNA binding protein.

tau Proteins↗

PubMed2025

Proteins that recognize unique features of U7 snRNA and may substitute for Gemin5 in the assembly of U7-specific Sm ring.

U7 snRNA is a 60 nucleotide component of U7 snRNP, a multisubunit endonuclease that cleaves precursors of metazoan replication-dependent histone mRNAs at the 3' end, hence generating mature histone mRNAs. The Sm site in U7 snRNA differs from the Sm site in spliceosomal snRNAs and promotes the assembly of a unique Sm ring containing Lsm10 and Lsm11 instead of the spliceosomal SmD1 and SmD2 proteins. While the spliceosomal-type Sm site is recognized by Gemin5, a subunit of the SMN complex, the identity of the protein that recognizes the unusual Sm site of U7 snRNA resulting in the incorporation of Lsm10 and Lsm11 has not been determined. Here, we looked for proteins in mammalian extracts that interact with U7 snRNA and identified polypyrimidine tract-binding protein 1 (PTBP1) and insulin-like growth factor 2 mRNA-binding protein 3 (IGF2BP3) as two major proteins with this characteristic. The binding of PTBP1 and IGF2BP3 to U7 snRNA depends on its unique Sm site and on the upstream CUCUUU motif that base-pairs with histone pre-mRNAs and defines substrate specificity of U7 snRNP. Among proteins that bind U7 snRNA, we also identified hnRNP A1. We show that hnRNP A1 interacts with the SMN protein of the SMN complex, a likely prerequisite for the protein that substitutes for Gemin5 in the assembly of U7-specific Sm ring. Our results also suggest a mechanism that explains why Gemin5 does not bind the Sm site of U7 snRNA.

RNA, Small Nuclear↗

PubMed2026

Histidine 62 in ArfB is required for stop codon-independent peptidyl-tRNA hydrolysis on the stalled ribosome.

Translating ribosomes can stall on mRNA for various reasons, including nuclease cleavage, arrest peptide sequences or ribosome collisions. In Escherichia coli, several ribosome rescue factors act to release ribosomes stalled at the 3' end of mRNA. Among these factors, ArfB can rescue stalled ribosomes without the help of other factors. ArfB consists of an N-terminal domain containing the catalytic GGQ motif, which mediates peptidyl-tRNA hydrolysis, and a C-terminal extension that functions as a sensor for recognizing stalled ribosomes. However, how these two regions coordinate to resolve ribosome stalling remains unclear. Here, using a reconstituted translation system, we found the functional importance of histidine residues in the N-terminal domain of ArfB. In particular, histidine at position 62 in E. coli ArfB was required for stop codon-independent fMet-tRNA hydrolysis, whereas its substitution did not affect affinity for the ribosome. Furthermore, directed hydroxyl radical probing revealed that H62A mutation did not significantly alter the overall positioning of either the N-terminal domain or the C-terminal extension of ArfB on the ribosome. These findings suggest that H62 contributes to ArfB function during a step following initial ribosome binding, thereby facilitating peptidyl-tRNA hydrolysis.

Ribosomes↗

PubMed2025

Mammalian antiviral proteins ZAP and KHNYN can independently restrict CpG-enriched avian viruses.

Zoonotic viruses are an omnipresent threat to global health. Influenza A virus (IAV) transmits between birds, livestock, and humans. Proviral host factors involved in the cross-species interface are well known. Less is known about antiviral mechanisms that suppress IAV zoonoses. We observed CpG dinucleotide depletion in human IAV relative to avian IAV. Notably, human ZAP selectively depletes CpG-enriched viral RNAs with its cofactor KHNYN. ZAP is conserved in tetrapods, but we uncovered that avian species lack KHNYN. We found that chicken ZAP may not affect IAV (PR8) or CpG-enriched IAV (PR8CG). Human ZAP or KHNYN independently restricted CpG-enriched IAV PR8CG by overexpression in chicken cells and by combined knockout in human cells. Additionally, mammalian ZAP-L and KHNYN also independently restricted an avian retrovirus (ROSV). Curiously, platypus KHNYN, the most divergent from eutherian mammals, was also capable of independent restriction of multiple diverse viruses. We suggest that some mammalian KHNYN can act as a bona fide restriction factor with cell-autonomous activity. Furthermore, we speculate that through repeated contact between avian viruses and mammalian hosts, protein changes may accompany CpG-biased mutations or reassortment to evade mammalian ZAP and KHNYN.

Animals↗

PubMed2009

Multipotent genetic suppression of retrotransposon-induced mutations by Nxf1 through fine-tuning of alternative splicing.

Cellular gene expression machinery has coevolved with molecular parasites, such as viruses and transposons, which rely on host cells for their expression and reproduction. We previously reported that a wild-derived allele of mouse Nxf1 (Tap), a key component of the host mRNA nuclear export machinery, suppresses two endogenous retrovirus-induced mutations and shows suggestive evidence of positive selection. Here we show that Nxf1(CAST) suppresses a specific and frequent class of intracisternal A particle (IAP)-induced mutations, including Ap3d1(mh2J), a model for Hermansky-Pudlak syndrome, and Atcay(hes), an orthologous gene model for Cayman ataxia, among others. The molecular phenotype of suppression includes approximately two-fold increase in the level of correctly-spliced mRNA and a decrease in mutant-specific, alternatively-processed RNA accumulating from the inserted allele. Insertional mutations involving ETn and LINE elements are not suppressed, demonstrating a high degree of specificity to this suppression mechanism. These results implicate Nxf1 in some instances of pre-mRNA processing, demonstrate the useful range of Nxf1(CAST) alleles for manipulating existing mouse models of disease, and specifically imply a low functional threshold for therapeutic benefit in Cayman ataxia.

Alternative Splicing↗

PubMed2024

Assessing nanobody interaction with SARS-CoV-2 Nsp9.

The interaction between SARS-CoV-2 non-structural protein Nsp9 and the nanobody 2NSP90 was investigated by NMR spectroscopy using the paramagnetic perturbation methodology PENELOP (Paramagnetic Equilibrium vs Nonequilibrium magnetization Enhancement or LOss Perturbation). The Nsp9 monomer is an essential component of the replication and transcription complex (RTC) that reproduces the viral gRNA for subsequent propagation. Therefore preventing Nsp9 recruitment in RTC would represent an efficient antiviral strategy that could be applied to different coronaviruses, given the Nsp9 relative invariance. The NMR results were consistent with a previous characterization suggesting a 4:4 Nsp9-to-nanobody stoichiometry with the occurrence of two epitope pairs on each of the Nsp9 units that establish the inter-dimer contacts of Nsp9 tetramer. The oligomerization state of Nsp9 was also analyzed by molecular dynamics simulations and both dimers and tetramers resulted plausible. A different distribution of the mapped epitopes on the tetramer surface with respect to the former 4:4 complex could also be possible, as well as different stoichiometries of the Nsp9-nanobody assemblies such as the 2:2 stoichiometry suggested by the recent crystal structure of the Nsp9 complex with 2NSP23 (PDB ID: 8dqu), a nanobody exhibiting essentially the same affinity as 2NSP90. The experimental NMR evidence, however, ruled out the occurrence in liquid state of the relevant Nsp9 conformational change observed in the same crystal structure.

Viral Nonstructural Proteins↗

PubMed2026

EWS-RNA Binding Protein 1: Structural Insights into Ewing Sarcoma by Conformational Dynamics Investigations.

BACKGROUND: Prior research has demonstrated that proteins play a significant role in the prognosis and treatments of various sarcomas, including Ewing sarcoma through the interplay of downstream signaling cascades. However, there is limited understanding about the strcucture conformation of EWSR1 and its structural implication in the prognosis of Ewsing Sarcoma by interaction with RNA molecules. AIMS: The primary goal of ongoing research is to determine how EWSR1 contributes to Ewing sarcoma. OBJECTIVE: The current study explores the complexity of EWSR1 structure and its conformational interactions with RNA in relation to Ewing sarcoma. METHODS: Here, we employed a comparative modeling approach to predict EWSR1 domains separately and assembled them into one structural unit using a DEMO server. Additionally, the RNA motifs interacting with EWSR1 were predicted, and the 3D model was built using RNAComposer. Protein-RNA docking and MD simulation studies were carried out to check the intermolecular interactions and stability behavior of docked EWSR1-RNA complexes. RESULTS: The overall results explore the structural insights into EWSR1 and their interactions with RNA, which may play a momentous role in co- and post-transcriptional regulation to control gene expression. CONCLUSION: Taken togather, our findings suggest that EWSR1 may be a useful therapeutic target for the diagnosis and management of Ewing sarcoma.

Sarcoma, Ewing↗

PubMed2026

Proximity labeling puts ZFP36L1 as central hub for posttranscriptional regulation networks in T cells.

Effective T cell responses against pathogens require a rapid yet tightly controlled remodeling of the proteome, and RNA binding proteins (RBPs) are key in this process. For instance, the RBP ZFP36L1 prevents excessive protein production and thereby limits immunopathology. ZFP36L1 is primarily known to mediate mRNA decay, but it can also regulate other processes. How its mode of action relates to its interaction partners is, however, not well-understood. Here, we mapped the ZFP36L1 interactome in primary human T cells. Using proximity labeling, we identified known and new interactors that regulate 3'UTR-mediated RNA degradation, deadenylation, stress granule/p-body formation, as well as 5'UTR-mediated translation repression and mRNA decapping. Snapshot analysis uncovered the ZFP36L1 interactome dynamics and RNA (in)dependency throughout T cell activation. Intriguingly, proximity labeling also uncovered regulators of ZFP36L1 protein expression. This included the helicase UPF1, which not only interacts with ZFP36L1 protein but that may also promote its protein expression. Altogether, this comprehensive interactome map underlines the versatility of interactions with ZFP36L1 and their possible role in cellular function.

Humans↗

PubMed1992

A regulatory cis element and a specific binding factor involved in the mitogenic control of murine ribosomal protein L32 translation.

The mRNA encoding ribosomal protein L32 redistributes from untranslated subribosomal particles into polysomes after mitogenic activation of quiescent T-lymphocytes and fibroblasts. To identify the regions of the L32 mRNA which are important in regulating its cytoplasmic location we constructed a plasmid containing the murine L32 cDNA under the control of the Rous sarcoma virus (RSV) long terminal repeat promoter and introduced this construct into murine 3T3 fibroblasts. The mRNA transcribed from the RSV-L32 construct redistributed from subribosomal particles into polysomes in response to mitogenic activation in a manner similar to endogenous L32 mRNA. A conserved polypyrimidine region present at the 5' terminus of all ribosomal protein mRNAs is required for translational regulation of L32 mRNA since deletion of this sequence resulted in a mRNA that was not sequestered in subribosomal particles in quiescent cells. A radioactive RNA probe containing the first 34 nucleotides of the L32 5'-untranslated region, including the polypyrimidine region, specifically interacted with a protein of about 56 kDa. This protein did not bind detectably to RNA probes lacking the polypyrimidine sequence. Binding activity was similar in protein extracts made from resting and activated cells, suggesting that binding of the 56-kDa protein as measured in this assay is not regulated. This protein is a member of what may be an emerging family of polyribopyrimidine-binding proteins with diverse biochemical functions.

3T3 Cells↗

PubMed1992

Cloning and expression of a cellular high density lipoprotein-binding protein that is up-regulated by cholesterol loading of cells.

Plasma membranes of cultured cells contain high affinity receptors for high density lipoprotein (HDL) that appear to mediate removal of excess intracellular cholesterol. Recent studies using ligand blot analysis have identified a 110-kDa membrane protein which has features predicted for an HDL receptor, in that it preferentially binds HDL apolipoproteins and undergoes up-regulation in response to cholesterol loading of cells. In this study, we isolated a cDNA clone from an expression library using an antibody raised against partially purified 110-kDa HDL-binding protein. This clone encodes a novel cell protein, designated HBP, comprised mostly of 14 imperfect tandem repeats of approximately 70 amino acids in length. Each repeat appears to contain two amphipathic helices. Expression of HBP in cultured cells was increased severalfold when cells were loaded with cholesterol, as evident by increases in both HBP mRNA and membrane-associated protein. Overexpression of HBP in mammalian cell transfectants was associated with higher HDL binding to isolated cell protein and with modest increases in HDL binding to the cell surface. Proteins identified by ligand blot analysis had lower apparent M(r) than the primary HBP gene product and varied in M(r) and in HDL binding activity between cell types, suggesting that HBP undergoes cell-specific processing. These results provide preliminary evidence that HBP is a component of a cellular pathway that facilitates removal of excess cholesterol from cells, perhaps through its interaction with HDL. However, the predicted structure of HBP does not conform to that of any known receptor, suggesting that it does not function as a classic plasma membrane receptor.

Amino Acid Sequence↗