Dynamics of Aspartate Isomerization and Its Cellular Implications: Evidence for Isoaspartate Formation in ER-Folded Proteins and Stress-Responsive Regulation of PCMT1.

Spontaneous isomerization of aspartate and deamidation of asparagine residues into isoaspartate (isoAsp) constitute major non-enzymatic post-translational modifications that alter protein structure, stability, and turnover. The repair enzyme protein L-isoaspartate O-methyltransferase (PCMT1) catalyzes methylation of isoAsp residues, thereby preventing their accumulation and preserving proteome integrity. Although PCMT1 has been studied extensively in cytoplasm and nucleus, its relationship to endoplasmic reticulum (ER) proteostasis remains poorly understood. Here, we investigated dynamics of aspartate isomerization within the cell, focusing on isoAsp accumulation and the regulation of PCMT1 localization under physiological and stress conditions. Using immunofluorescence, subcellular fractionation, and in vitro methylation assays, we detected isoAsp-modified proteins within the ER-enriched fractions of HeLa cells. We found that ER stress induction enhanced formation of isoAsp-containing proteins, with MG132 treatment producing the highest accumulation. PCMT1 expression increased under both stress conditions, accompanied by distinct subcellular redistribution between the cytoplasmic and nuclear compartments. These observations indicate that ER-folded proteins are susceptible to spontaneous aspartate isomerization, and that PCMT1 activity dynamically responds to proteostatic stress. Our findings provide the first experimental evidence linking isoAsp formation within the ER to PCMT1-mediated protein repair, thereby integrating chemical instability with cellular quality-control pathways. This study establishes a structural and cellular framework for understanding the dynamics of aspartate isomerization in the cell and underscores significance of PCMT1 in maintaining proteostasis under stress conditions.