A distinct subclass of mammalian striated myosins: structure and molecular evolution of "superfast" or masticatory myosin heavy chain.

"Superfast" or masticatory myosin is the molecular motor in the powerful and specialized jaw-closing muscles of carnivores, folivores, and frugivores. This myosin presumably underpins the unusual high force and moderate shortening velocity of muscle fibers expressing it. Here, we report the cloning and sequencing of the cDNA encoding the full-length masticatory myosin heavy chain (MyHC) from cat temporalis muscle. This was obtained by immunoscreening a cDNA expression library and RACE-PCR (rapid amplification of cDNA ends-PCR). Sequence comparisons at the DNA and amino acid levels show that masticatory MyHC has less than 70% homology to known striated MyHCs, compared with 87-96% between other mammalian fast isoforms themselves. Nucleotide substitution rates at the nonsynonymous sites between masticatory MyHC and other mammalian striated MyHCs are considerably higher than between these striated MyHCs themselves. Phylogenetic analysis revealed that masticatory MyHC diverged from invertebrate MyHC before the avian cardiac MyHC subclass and the mammalian fast/developmental and slow/cardiac MyHC subclasses. Masticatory MyHC is thus a distinct new subclass of vertebrate striated myosins. The early divergence from invertebrate MyHC, combined with immunochemical evidence of its expression in reptilian and shark jaw-closing muscles, suggests that masticatory MyHC evolved in early gnathostomes, driven by benefits derived from powerful jaw closure. During the mammalian radiation, some taxa continued to express it, while others adapted to new types of food and eating habits by replacing masticatory MyHC with more appropriate isoforms normally found in limb and cardiac muscles.