THE TYPE-II ISOFORM OF BOVINE BRAIN PROTEIN L-ISOASPARTYL
METHYLTRANSFERASE HAS AN ENDOPLASMIC-RETICULUM RETENTION SIGNAL (...RDEL) AT ITS C-TERMINUS
by POTTER-SM JOHNSON-BA HENSCHEN-A ASWAD-DW (*R) GUZZETTA-AW
UNIV CALIF IRVINE,SCH BIOL SCI,DEPT PSYCHOBIOL/IRVINE//CA/92717 (*R)
BIOCHEMISTRY
VOL: 31 (27):6339-6347(1992)
LOC: CHEM KERC
Abstract:
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.