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PubMed1989

The primary structure of a protein carboxyl methyltransferase from bovine brain that selectively methylates L-isoaspartyl sites.

Protein L-isoaspartyl methyltransferase (PIMT) transfers the methyl group of S-adenosyl-L-methionine to free alpha-carboxyl groups of atypical L-isoaspartyl residues in proteins. The complete primary structure of the type I isoform of bovine brain PIMT was determined by sequence analysis of peptides generated by endoprotease Lys-C, trypsin, cyanogen bromide, and endoprotease Asp-N digests. The correct composition of every peptide was verified by fast atom bombardment mass spectrometry. The efficiency of sequencing by tandem mass spectrometry was examined for several peptides by comparing its speed and accuracy with automated Edman degradation. Tandem mass spectrometry was used to determine the structure of the NH2-terminal blocked peptide derived from a hydroxylamine cleavage. PIMT is 226 residues with Mr = 24,500 and contains acetyl alanine as the amino-terminal residue. The partial sequence (141 residues from 8 tryptic peptides) of a homologous human red cell PIMT (Gilbert, J. M., Fowler, A., Bleibaum, J., and Clarke, S. (1988) Biochemistry 27, 5227-5233) shows a 97% identity with the corresponding peptides of the bovine brain enzyme. The complete brain enzyme sequence reported here bears no significant homology to any other known class of methyltransferase including those which methylate the side chain gamma-carboxyl group of receptor proteins involved in bacterial chemotaxis.

Amino Acid Sequence↗
PubMed1992

The type II isoform of bovine brain protein L-isoaspartyl methyltransferase has an endoplasmic reticulum retention signal (...RDEL) at its C-terminus.

Bovine brain is known to contain two major isoforms of protein L-isoaspartyl methyltransferase (PIMT), an enzyme that facilitates repair of atypical L-isoaspartyl peptide bonds in proteins. Although the two isoforms can be separated by anion-exchange chromatography, they appear to have similar, if not identical, substrate specificities in vitro. The more basic type I isoform has been extensively characterized, and its complete sequence has been reported. The present study was undertaken in an attempt to understand the structural and functional uniqueness of the more acidic type II isoform. Electrospray mass spectrometry of the intact enzymes revealed that the type II isoform is approximately 43 amu heavier than the type I isoform. Cyanogen bromide cleavage followed by HPLC with on-line mass analysis revealed that the type II isoform contains a unique C-terminal fragment which is 43 amu heavier than the corresponding fragment from the type I isoform. Amino acid composition analysis and direct sequencing of this fragment indicate that the type II isoform ends in the sequence ...RDEL, while the type I is known to end in ...RWK. Since ...RDEL, like ...KDEL, serves as an effective endoplasmic reticulum retention signal, we propose that the type II isoform serves to repair damaged proteins within the endoplasmic reticulum or, perhaps, within some other specialized compartment of the cell. Comparison of the protein sequences of the two bovine brain isoforms to DNA sequences for rodent PIMT reported by others suggests that the type II isoform may be produced by splicing within the codon for Arg224.

Amino Acid Sequence↗
PubMed2025

The rice heat shock transcription factor OsHSFC1b increases seed weight, size, and vigor, but its function is disrupted by isoaspartyl modification.

Plant optimizes seed size, weight, vigor, and various other features during seed development, which are important not only for their successful propagation and establishment but also for effective agriculture. Despite several studies conducted, understanding how plants coordinate the regulatory mechanisms to achieve optimal seed size, weight, and vigor remains elusive. Here, our study reveals the role of rice heat shock transcription factor OsHSFC1b in modulating various seed attributes. We observe that OsHSFC1b expression increases during the later stage of seed development and is primarily localized in the embryo. We found that hsfc1b genome-edited lines exhibit compromised seed size, weight, and vigor, while overexpression lines exhibit increased seed size, weight, and vigor compared with the wild-type seeds. Our study further reveals that OsHSFC1b improves seed vigor by activating HSPs and RFO biosynthetic genes involved in protection mechanisms, while also mediating seed size and weight by modulating auxin biosynthesis, endosperm development, and seed filling. We found that upon ageing and stressful environments, OsHSFC1b undergoes isoaspartyl modification that negatively impacts its biological function in seeds. Our MS/MS analyses confirm that asparagine residues near the DNA-binding domain and nuclear localization sequence of OsHSFC1b undergo isoaspartyl modification that adversely affects OsHSFC1b's transactivation activity. However, PROTEIN L-ISOASPARTYL METHYLTRANSFERASE interacts and repairs this isoaspartate-mediated damage, and restores the function of OsHSFC1b. Taken together, our study uncovers how isoaspartyl modification affects the transactivation ability of OsHSFC1b, yet the intervention of PIMT not only repairs this damage but also elevates agronomically important seed traits.

Oryza↗