Variations in apparent mass of mammalian fast-type myosin light chains correlate with species body size, from shrew to elephant.

A recent study (Bicer S and Reiser PJ. J Muscle Res Cell Motil 25: 623-633, 2004) suggested considerable variation in the apparent molecular mass (M(a)), deduced from electrophoretic mobility, in fast-type myosin light chains (MLCF), especially MLC1F, among mammalian species. Furthermore, there was an indication that MLC1F M(a) generally correlates with species body mass, over an approximately 4,000-fold range in body mass. The results also suggested that M(a) of other low-molecular-weight myofibrillar proteins is less variable and not as strongly correlated with body mass among the same species. The objective of this study was to test the hypotheses that the M(a) of MLCs does, in fact, vary and correlate with species body mass. The electrophoretic mobilities of MLCF isoforms from 19 species, varying in size approximately 500,000-fold, were quantitated. The results confirm that the M(a) of MLC1F and MLC2F vary significantly among mammals, spanning a very broad range in body mass; the MLC1F M(a) varies more than that of other low-molecular-weight myofibrillar proteins; and there is a significant correlation between species body mass and MLC1F M(a). Differences in MLC1F M(a) among five species can be accounted for by differences in the reported amino acid sequence, especially the length of a common polyalanine region near the NH(2)-terminal actin-binding site. The possibility that the differences in MLC1F sequence among mammalian species, in and adjacent to the actin-binding region, are related to differences in modulation of cross-bridge kinetics in species with diverse locomotion kinetics is discussed.