Quantitative Phosphoproteomics Identifies Myofibrillar Protein Phosphorylation Mediated by Pyruvate Kinase M2 in Beef

Pyruvate kinase M2 (PKM2) influences meat quality through glycolysis and also exhibits its moonlighting function as a protein kinase that catalyzes protein phosphorylation. However, it remains unclear whether PKM2 phosphorylates myofibrillar proteins, thereby affecting postmortem myofibrillar protein stability. This study investigates PKM2's non-canonical kinase function using quantitative phosphoproteomics and an in vitro myofibrillar protein incubation model to identify its phosphorylation substrates and functional impacts. The quantitative phosphoproteomics identified 441 phosphoproteins, 881 phosphopeptides, and 1040 phosphorylation sites. Notably, the myosin regulatory light chain (MRLC) was identified as a likely candidate phosphorylation substrate of PKM2 in vitro. The interaction between PKM2 and MRLC was confirmed using co-immunoprecipitation (Co-IP) and Western blotting. Furthermore, MRLC phosphorylation by PKM2 significantly inhibited its degradation and enhanced its stability. This work establishes an in vitro biochemical framework for the moonlighting role of glycolytic enzymes, suggesting a potential mechanistic pathway that might influence myofibrillar protein stability during meat aging.