The objective of this proposal is to understand the physiological significance of an enzymatic methylation reaction which appears to be a step in the metabolism of altered intracellular proteins. This ubiquitous enzymatic activity catalyzes the S- adenosylmethionine-dependent methyl esterification of isomerized and racemized aspartic acid residues in a wide variety of membrane and cytosolic proteins. Normally functional L- aspartyl and L-asparaginyl residues can be altered by spontaneous aging and/or damage reactions to form D-aspartyl residues and L- isoaspartyl transpeptidation products. Proteins containing these latter residues may not be expected to be fully functional, and we are interested in pursuing the possibility that the enzymatic methylation of these proteins may lead to their repair and/or degradation. Since the chemical deterioration, replacement, or repair of various macromolecules is likely to be a major determinant in the aging process of an organism, understanding the role of these reactions may contribute significantly to our concepts of the stability of cellular physiological systems over extended periods of time. We propose to concentrate our efforts on the methylation of abnormal proteins in the human erythrocyte system, where the absence of protein synthesis simplifies the experimental design. Although previous work has shown that similar methylation reactions occur in other mammalian tissues, differences in the metabolism of the methylated proteins may exist and we will be interested in comparing these reactions in non-erythroid tissues. Our specific goals in this project period include furthering our understanding of the enzymology of protein carboxyl methyltransferases, characterizing the specific sites of methylation on membrane and cytosolic proteins in the cell, and delineating the various possible pathways for the metabolism of methylated proteins. We will continue to use synthetic peptides as models for the methylated and demethylation reactions. Our long term goal will be to understand the functional role of protein covalent modification reactions that are involved in the repair and/or specific degradation of cellular proteins.
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