Top-down Mass Spectrometry of Sarcomeric Protein Post-translational Modifications from Non-human Primate Skeletal Muscle

Sarcomeric proteins, including myofilament and Z-disk proteins, play critical roles in regulating muscle contractile properties. A variety of isoforms and post-translational modifications (PTMs) of sarcomeric proteins have been shown to be associated with modulation of muscle functions and the occurrence of muscle diseases. Non-human primates (NHPs) are excellent research models for sarcopenia, a disease associated with alterations in sarcomeric proteins, due to their marked similarities to humans. However, the sarcomeric proteins in NHP skeletal muscle have not been well characterized. To gain a deeper understanding of sarcomeric proteins in NHP skeletal muscle, we employed top-down mass spectrometry (MS) to conduct a comprehensive analysis on isoforms and PTMs of sarcomeric proteins in rhesus macaque skeletal muscle. We identified 23 protein isoforms with 46 proteoforms of sarcomeric proteins, including 6 isoforms with 18 proteoforms from fast skeletal troponin T. Particularly, for the first time, a novel PDZ/LIM domain protein isoform, PDLIM7, was characterized with a newly identified protein sequence. Moreover, we also identified multiple PTMs on these proteins, including deamidation, methylation, acetylation, tri-methylation, phosphorylation, and S-glutathionylation. Most PTM sites were localized, including Asn13 deamidation on MLC-2S; His73 methylation on aactin; N-terminal acetylation on most identified proteins; N-terminal tri-methylation on MLC-1S, MLC-1F, MLC-2S, and MLC-2F; Ser14 phosphorylation on MLC-2S; and Ser15 and Ser16 phosphorylation on MLC-2F. In summary, a comprehensive characterization of sarcomeric proteins including multiple isoforms and PTMs in NHP skeletal muscle was achieved by analyzing intact proteins in the top-down MS approach.