L-isoaspartyl methyltransferase

protein
Name=Protein-L-isoaspartate(D-aspartate) O-methyltransferase
caption=


width=
HGNCid=8728
Symbol=PIMT
AltSymbols=
EntrezGene=PCMT1
OMIM=176851
PDB=
ECnumber=2.1.1.77
Chromosome=6
Arm=q
Band=22.3-24
LocusSupplementaryData=

Protein L-isoaspartyl methyltransferase or (PIMT) (EC number|2.1.1.77), systematic name "(S-adenosyl-L-methionine:protein-L-isoaspartate O-methyltransferase)" is an enzyme which recognizes and catalyzes the repair of damaged L-isoaspartyl and D-aspartate groups in proteins. It is a highly conserved enzyme which is present in nearly all eukaryotes, archaebacteria, and Gram-negative eubacteriaRyttersgaard, Carsten, and Yeates, Todd O. (2002) Crystal Structure of Human L-Isoaspartyl Methyltransferase. J. Biol. Chem., 277, 10642-6. Entrez Pubmed|11792715] .

Function

PIMT acts to transfer methyl groups to side chain carboxyl groups of L-isoaspartyl and D-aspartate groups, converting these abnormal proteins to the normal L-aspartate. The enzyme takes the end methyl residue from the methionine side chain and adds it to the side chain carboxyl group of L-isoaspartate or D-aspartate to create a methyl ester [web cite|url=http://www.chem.qmul.ac.uk/iubmb/enzyme/EC2/1/1/77.html|title=IUBMB Enzyme Nomenclature] . This catalyzes a transformation to L-succinimide, which is a precursor to aspartate and isoaspartate. PIMT tends to act on proteins that have been non-enzymatically damaged due to age. By performing this repair mechanism, the enzyme helps to maintain overall protein integrity. This mechanism has been observed by several independent research groups, and has been confirmed through experimental testing. In one report, PIMT was inhibited by adenosine dialdehyde. The results supported the proposed function of the enzyme, as the amount of abnormal L-aspartate residues increased when cells were treated with the indirect inhibitor, adenosine dialdehydeJohnson BA, and Aswad, DW. (1993) Accumulation of substrates for protein L-isoaspartyl methyltransferase in adenosine dialdehyde-treated PC12 cells. J. Biol. Chem., 268, 6174-81. Entrez Pubmed|8454593] . Additionally, S-adenosylhomocysteine is known to be a competitive inhibitor of PIMTJohnson BA, and Aswad, DW. (1993) Accumulation of substrates for protein L-isoaspartyl methyltransferase in adenosine dialdehyde-treated PC12 cells. J. Biol. Chem., 268, 6174-81. Entrez Pubmed|8454593] . When PIMT is not present in cells, the abnormal aspartyl residues accumulate, creating abnormal proteins that have been known to cause fatal progressive epilepsy in miceYamamoto A, Takagi H, and Shirasawa T. (1998) Deficiency in protein L-isoaspartyl methyltransferase results in a fatal progressive epilepsy. J Neurosci. 18(6):2063-74. Entrez Pubmed|9482793] . It has been suggested that calmodulin may play a role in stimulating the function of PIMT, although the relationship between these two molecules has not been thoroughly explored O’Connor, MB and O’Connor, CM. (1998) Complex interactions of the protein L-isoaspartyl methyltransferase and calmodulin revealed with the yeast two-hybrid system. J. Biol. Chem., 273, 12909-13. Entrez Pubmed|9582322 ] . In addition to calmodulin, guanosine 5'-O- [gamma-thio] triphosphate (GTPgammaS) has been found to stimulate PIMT activityBilodeau, D and Beliveau R. (1999) Inhibition of GTPgammaS-dependent L-isoaspartyl protein methylation by tyrosine kinase inhibitors in kidney. Cell Signal., 11, 45-52. Entrez Pubmed|10206344] .

tructure

The enzyme is present in human cytosol in two forms due to alternative splicing and differs among individuals in the population due to a single polymorphism at protein 119, either valine or isoleucine. The enzyme structure is described as a “doubly wound alpha/beta/alpha sandwich structure” which is quite consistent in all species analyzed thus farRyttersgaard, Carsten, and Yeates, Todd O. (2002) Crystal Structure of Human L-Isoaspartyl Methyltransferase. J. Biol. Chem., 277, 10642-6. Entrez Pubmed|11792715] . If there is any difference in the sequences between different organisms it occurs in the regions connecting the three motifs in the sandwich structure, but the sequence of the individual motifs tends to be highly conserved. Researchers have found the active site to be in the loop between the beta structure and the second alpha helix and have determined it to be highly specific for isoaptartyl residues. For example, the residues found at the C-terminus of drosophila PIMT (dPIMT) are rotated 90 degrees so as to allow more space for a substrate to interact with the enzyme. In fact, dPIMT appears to alternate between this unique open conformation and the less open conformation common of PIMT in other organisms. Although possibly unrelated to this, increased levels of dPIMT in drosophila have been correlated with increase life expectancy in these organisms due to their importance in protein repairBennett, EJ and O'Connor, CM. (2003) Catalytic implications from the Drosophila protein L-isoaspartyl methyltransferase structure and site-directed mutagenesis. Biochemistry,42, 12844-53. Entrez Pubmed|14596598] .

Reaction

ee also

* methyltransferase
* isoaspartate
* aspartic acid

External links

*
* [http://www.rcsb.org/pdb/explore.do?structureId=1KR5 Enzyme Entry On ProteinDatabank]

References