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Biomedical subjects

C N Chang

Publications and source records attributed to C N Chang.

122 records · Page 7Linked to original sources
PubMed1979

Membrane biogenesis: cotranslational integration of the bacteriophage f1 coat protein into an Escherichia coli membrane fraction.

The coat protein (CP) of bacteriophage f1 is integrated into an Escherichia coli plasma membrane fraction consisting of inverted vesicles when it is synthesized in a cell-free, coupled transcription--translation system supplemented with the inverted vesicles. By using proteolytic enzymes as probes, we found by subsequent peptide mapping and determination of the sequence of the proteolytic products that CP was inserted into the inverted vesicles in an orientation indistinguishable from that in inverted vesicles prepared from infected E. coli: only a COOH-terminal portion of approximately 10 residues was accessible to proteolysis, whereas the remainder of CP (CP') was entirely protected. Protection of CP' was dependent on the integrity of the vesicle membrane, because it was abolished when proteolysis was done in the presence of nonionic detergents. Insertion was observed when the inverted vesicles were present during translation in the cell-free system, not when they were added after translation. Thus, the asymmetric insertion of this type of integral membrane protein is strictly coupled to translation. These findings are discussed with respect to prokaryotic membrane biogenesis and are related to bacteriophage f1 assembly and infection.

Bacterial Proteins↗
PubMed1979

Translocation of proteins across membranes: the signal hypothesis and beyond.

Proteins are translocated across membranes either coupled to translation (co-translationally) or after translation (post-translationally). The information for both modes of translocation is encoded in the protein in the form of a short-lived sequence extension (signal sequence). Additional information resides in the ribosome in the case of co-translational translocation, which proceeds via a ribosome--membrane junction. Translocation is mediated by specific receptors (ribosome and/or signal receptors) which are restricted in their location to distinct cellular membranes. In most cases the signal sequence is removed by a signal peptidase operating in an endoproteolytic mode. Membranes endowed with receptors for co-translational translocation are: the rough endoplasmic reticulum (RER) including the outer nuclear envelope membrane, the inner mitochondrial membrane and the thylakoid membrane of chloroplasts, in eukaryotic cells; and the plasma membrane in prokaryotic cells. Each of these membranes presumably contains a single distinctive signal receptor, ribosome receptor and signal peptidase. Membranes endowed with one distinct receptor each for post-translational translocation are both mitochondrial membranes, the chloroplast envelope membrane and the peroxisomal membrane. A signal sequence for co-translational translocation across the RER membrane that is identical in its secondary structure is shared by secretory, lysosomal and certain bitopic integral membrane proteins. Some integral membrane proteins presumably share another common sequence--referred to as stop-transfer sequence--which serves to interrupt translocation and thereby to orient the polypeptide chain in the lipid bilayer. Furthermore, the existence of a few specific 'sorting' sequences is postulated. These would be common to many proteins and would serve to route them to their final destination following translocation across or orientation within the membrane. Thus, the topological information which determines the intracellular pathway and the final location of a great number of proteins appears to reside in a small repertoire of specific sequences which are either a transient or a permanent part of the protein.

Animals↗
PubMed1978

Detection of prokaryotic signal peptidase in an Escherichia coli membrane fraction: endoproteolytic cleavage of nascent f1 pre-coat protein.

An inverted membrane vesicle fraction isolated from uninfected Escherichia coli and largely derived from the inner membrane has been shown to contain an endoproteolytic activity that cleaves nascent bacteriophage f1 pre-coat protein into two identifiable products. The electrophoretic mobility on sodium dodecyl sulfate/urea/polyacrylamide gels and the partial amino-terminal sequence of the larger fragment were indistinguishable from those of the mature phage coat protein. Partial amino-terminal sequence analysis showed that the smaller fragment corresponds to the amino-terminal "signal peptide" of f1 pre-coat protein. Cleavage occurred only if the membrane fraction was present during in vitro synthesis, and was not observed if it was added after completion of pre-coat protein synthesis. The cleavage reaction was strongly stimulated when the membrane fraction was present together with the nonionic detergent Nikkol. These results are consistent with and discussed in terms of the signal hyothesis.

Amino Acid Sequence↗
PubMed1975

Purification and properties of a ribosomal protein methylase from Eschericha coli Q13.

Ribosomal protein methylase has been purified from Escherichia coli strain Q13 using methyl-deficient 50S subunits as substrates. The purified enzyme (or enzyme complex) which is devoid of rRNA methylating activity is quite stable and has a pH optimum around 8.0. The Km for S-adenosyl-L-methionine is 3.2 muM. The molecular weight of the enzyme is 3.1 X 10(4); minor methylating activity was also detected for protein peaks with molecular weights of 1.7 X 10(4) and 5.6 X 10(4). Protein L11 is the major protein methylated by the purified enzyme. Product analysis revealed the presence of N epislon-trimethyllysine, a methylated neutral amino acid(s) previously observed in protein L11 and N epislon-monomethyllysine. Free ribosomal proteins were much better substrates for the methylation, indicating that methylation of 50S ribosomal proteins can occur before the complete assembly of the 50S ribosomal subunit.

Escherichia coli↗
PubMed1975

Methylation of the ribosomal proteins in Escherichia coli. Nature and stoichiometry of the methylated amino acids in 50S ribosomal proteins.

Methylated ribosomal proteins from Escherichia coli 50S subunit are localized by growing cells in a medium containing (1-14C)methionine and (3H-methyl)-methionine and comparing the 3H/14C ratio for each of the 50S ribosomal proteins. The following proteins are methylated: L11, L1, L3, L5, L7, L8, L9, L12, L18, and L33. The nature and stoichiometry of the methylated amino acid(s) in each of the methylated proteins are determined. Protein L11 is the most heavily methylated of all the 50S subunit proteins. This protein has previously been implicated in the peptidyl transferase reaction during protein synthesis (K. H. Nierhaus and V. Montejo (1973), Proc. Nat. Acad. Sci. U. S. 47, 1588-1602). Three proteins (L1, L3, and L5) have intermediate levels of methylation and contain about 0.4-0.6 methyl groups each per molecule of protein. Five other proteins (L7, L8, L9, L12, and L18) are also methylated to a slight extent (-0.1 methyl group/molecule of protein). One unknown methylated neutral amino acid was detected in protein L11 and at least one and possibly two other unidentified methylated amino acids appeared to be present in protein L33.

Amino Acids↗
PubMed1974

Methylation of ribosomal proteins in Escherichia coli.

Escherichia coli was grown in a medium containing [1-(14)C]methionine and [methyl-(3)H]methionine, and the (3)H/(14)C ratio was determined for each of the ribosomal proteins derived from the 70S ribosome. Evidence indicates that six proteins from the 50S subunit were methylated: L7, L9, L11, L12, L18, and L33. Methylation of several other 50S proteins (such as L1, L3, L5, etc.) may also occur. The methylated amino acids in protein L11 have been characterized further and found to be predominately epsilon-trimethyllysine. A small amount of a compound tentatively identified as N(G), N'(G)-dimethylarginine was also detected.

Amino Acids↗
PubMedDate not supplied

Serial MRI studies in pathologically verified Balò's concentric sclerosis.

We report serial MRI studies in a patient with Balò's concentric sclerosis, who presented with a progressive weakness in the right limbs of 10 day duration. Serial MR images revealed several concentric lesions characterized by layer by layer of demyelinated and myelinated appearance and multiple plaques in the white matter. Pathological study of a biopsy of a concentric lesion (which was performed to rule out neoplasm) further confirmed this diagnosis. These MR findings resolved in correspondence with clinical improvement. The patient was almost completely normal 1 year later. In conclusion, in this case, we found that MRI could show the striking feature reminiscent of the histopathologic findings of Balò's concentric sclerosis and played an important role in antemortem diagnosis of this rare demyelinating disease.

Adult↗
PubMedDate not supplied

Seizures of fronto-orbital origin: a proven case.

Distinguishing characteristics of seizures of frontal origin have not been clearly delineated. We describe a case of seizures of proven fronto-orbital origin to provide further definition of characteristics of seizures arising in that lobe. A 36-year-old man had medically intractable seizures since age 8 years. Clinically the seizures were stereotyped, with cessation of activity followed by turning of head and body to the right and then by struggling, kicking, and vocalizations indicating fear. Total seizure duration was approximately 30 s, with an apparent abrupt return of consciousness. The interictal scalp EEGs were similar to those of primary generalized epilepsy, with bisynchronous though asymmetric epileptiform activity anteriorly. Ictal scalp recordings were not localizing. Recordings from bilateral frontal and temporal subdural electrodes showed an electrical focus in the right fronto-orbital region which was confirmed by electrocorticography (ECoG) at the time of craniotomy. The right fronto-orbital cortex was resected and on pathologic examination showed gliotic tissue and intracytoplasmic neuronal inclusions of periodic acid-Schiff (PAS)-positive granules consistent with lipofuscinosis. The patient has remained seizure-free for 6 years after operation.

Adult↗