Reference "Characterization of plant L-isoaspartyl methyltransferases that may be involved in seed survival: purification, cloning, and sequence analysis of the wheat germ enzyme"

Reference ID

9181

Title

Characterization of plant L-isoaspartyl methyltransferases that may be involved in seed survival: purification, cloning, and sequence analysis of the wheat germ enzyme

Source

Biochemistry (John Wiley & Sons), 1993, vol. 32, pp. 11100-11111

Authors (2)

Mudgett-M-B

Clarke-S

Abstract

Protein carboxyl methyltransferases (EC 2.1.1.77) that catalyze the transfer of
a methyl group from S-adenosylmethionine to L-isoaspartyl and D-aspartyl
residues in a variety of peptides and proteins are widely, but not universally,
distributed in nature. These enzymes can participate in the repair of damaged
proteins by facilitating the conversion of abnormal L-isoaspartyl residues to
normal L-aspartyl residues. In this work, we have identified L-isoaspartyl
methyltransferase activity in a variety of higher plant species and a green
alga. Interestingly, the highest levels of methyltransferase were located in
seeds, where the problem of spontaneous protein degradation may become
particularly severe upon aging. The wheat germ methyltransferase was purified as
a monomeric 28,000-Da species by DEAE-cellulose chromatography, reverse ammonium
sulfate gradient solubilization, and gel filtration chromatography. The purified
enzyme recognized a variety of L-isoaspartyl-containing peptides, but did not
recognize two D-aspartyl-containing peptides that are substrates for the
mammalian enzyme. The partial amino acid sequence was utilized to design
oligonucleotides to isolate a full-length cDNA clone, pMBM1. Its nucleotide
sequence demonstrated an open reading frame encoding a polypeptide of 230 amino
acid residues with a calculated molecular weight of 24,710. This sequence shares
31% identity with the L-isoaspartyl methyltransferase from Escherichia coli and
50% identity with the L-isoaspartyl/D-aspartyl methyltransferase from human
erythrocytes. Such conservation in sequence is consistent with a fundamental
role of this enzyme in the metabolism of spontaneously damaged polypeptides.