Structure and Function of Myosin Isoforms in Adult Chicken Hindlimb Muscles

Although large accumulation of sequence data is published for a variety of myosin heavy chain (MHC) isoforms, the meaning of heterogeneity among the amino acid sequences remains unclear as to the key contractile and biochemical properties of muscle fibres. In the present study, we studied on MHC isoforms in three adult chicken hindlimb muscles: ilio-tibialis, gastrocnemius and femoritibialis) and pectoralis muscle, by means of in vitro motility assay and measurement of ATPase activity. The motility speed of myosins and ATPase activities of myofibrils extracted from the hindlimb muscles were significantly lower than those from the pectoralis muscle consisting of a homogeneous MHC (P-type). ATPase activity of femori-tibialis myofibril was remarkably lower than those of ilio-tibialis and gastrocnemius myofibrils. We found the differential expression of MHC isoforms in these muscles by northern blot analysis. Furthermore, we determined the amino acid sequences of the 23kDa, 50kDa and 20kDa fragments from a major MHC isoform (G-type) found in the three hindlimb muscles. There was approximately 4.3% amino acid difference between G-type and P-type, however the characteristically methylated amino acids were recognized in the G-type at the same residues as in the P-type. In the course of sequencing the 20kDa fragment from femori-tibialis muscle myosin, we found another MHC isoform (F-type). Contentratios of P-type, G-type and F-type were about 3 : 7 : 0 in ilio-tibialis, 2 : 7 : 0 in gastrocnemius, and 1 : 6 : 3 in femori-tibialis, respectively. All these data suggest that the motility speed of myosin and ATPase activity of myofibril correlate with the content-ratio of the MHC isoforms in each muscle