Protein variants of skeletal muscle regulatory myosin light chain isoforms: prevalence in mammals, generation and transitions during muscle remodelling.

The regulatory myosin light chains (rMLCs) of mammalian skeletal muscle display protein diversity arising from the existence of different isotypes and protein phosphorylation. Two-dimensional electrophoresis and immunoblotting allowed us to identify three variants of the slow and fast rMLC isoforms (designated LC2s, LC2s1, LC2s2, and LC2f, LC2f1, LC2f2, respectively, from less to more acidic). This study aimed to characterize their prevalence among different species and muscle types, the mechanism(s) of their generation and their transitions during fast-to-slow fibre type switching. In vitro dephosphorylation and back-phosphorylation experiments and mass spectrometric analysis of tryptic digests indicate that: (1) both acidic variants, within each isoform, contain a phosphorylated peptide, (2) all variants of each isoform share identical tryptic peptides, (3) only one phosphopeptide is present per isoform, and (4) the intermediate-acidic variants of both isoforms contain the same peptide in their phosphorylated and non-phosphorylated forms. The data indicate that the triad pattern of variants results from two partially superimposed doublets of phosphorylated/non-phosphorylated pairs. Continuous, low-frequency electrical stimulation of rat extensor digitorum longus muscle changed the relative proportions of variants within each isoform towards those of the soleus. It is suggested that the doublets of phosphorylated/non-phosphorylated pairs are involved in rMLC exchange during sarcomere remodelling.