Slow skeletal muscle actin

Abstract

It had been thought that vertebrates synthesize the same isoform of sarcomeric actin in all skeletal muscles. The present work demonstrates the existence of a unique variant which accounts for all of the sarcomeric actin in the slow skeletal trunk muscle of Atlantic herring. Residues 48–375 of the primary structure have been inferred from nucleotide sequencing (Acc# EF495203) and residues 48–61 confirmed by Edman based sequencing of a fragment generated by subtilisin cleavage. EF495203 differs from the same segment of slow skeletal muscle actin from salmonids (Mudalige et. al. FEBS J. (2007) 274, 3452-3461) by a single residue (# 353), but there are eleven and ten substitutions, respectively, between EF495203 and salmon fast skeletal actin and rabbit skeletal actin. At least half of these substitutions are of a non-conservative nature. Actins isolated from different skeletal muscles from herring and salmon, but not rabbit, chicken and frog, can be differentiated by electrophoretic mobility at alkaline pH in the presence of 8M urea; digestion with various proteases, including thrombin, subtilisin and V8, and resistance to induced-denaturation. The melting temperatures of various G-actins (Ca.ATP) are: ∼45 (salmon slow skeletal muscle); ∼50 (herring slow skeletal muscle) and ∼55 degrees C (salmon, herring and rabbit fast skeletal muscle). Possible sources of the enhanced chain flexibility will be discussed. The demonstration of slow skeletal muscle actin in two unrelated teleosts indicates that it is not a lone occurrence.