Alternative exon-encoding regions of Locusta migratoria muscle myosin modulate the pH dependence of ATPase activity

Whereas the vertebrate muscle myosin heavy chains (MHCs) are encoded by a family of Mhc genes, most insects examined to date contain a single Mhc gene and produce all of the different MHC isoforms by alternative RNA splicing. Here, we found that the migratory locust, Locusta migratoria, has one Mhc gene, which contains 41 exons, including five alternative exclusive exons and one differently included penultimate exon, and potentially encodes 360 MHC isoforms. From the adult L. migratoria, we identified 14 MHC isoforms (including two identical isoforms): four from flight muscle (the thorax dorsal longitudinal muscle), three from jump muscle (the hind leg extensor tibiae muscle) and seven from the abdominal intersegmental muscle. We purified myosins from flight muscle and jump muscle and characterized their motor activities. At neutral pH, the flight and the jump muscle myosins displayed similar levels of in vitro actin-gliding activity, whereas the former had a slightly higher actin-activated ATPase activity than the latter. Interestingly, the pH dependences of the actin-activated ATPase activity of these two myosins are different. Because the dominant MHC isoforms in these two muscles are identical except for the two alternative exon-encoding regions, we propose that these two alternative regions modulate the pH dependence of L. migratoria muscle myosin.