Structural and functional variations in skeletal-muscle and scallop muscle actins

Structural and functional properties in two striated-muscle actins, one from a vertebrate, the other from an invertebrate (scallop), were compared in relation to a smooth-muscle actin isoform (aortic actin). In spite of differences in the variable N-terminal region, the two striated-muscle isoactins showed, in contrast with aortic actin, a large structural homology revealed by proteinase-susceptibility and interaction with the myosin head. Thus the myosin head may bind to the two striated-muscle actins in constant parts of the 18-113 sequence. In contrast, antigenic reactivity of conformational epitopes of these actins strongly differentiated scallop actin from the two others. The behaviour of the scallop actin appears to be related to several amino acid substitutions located near or at functional domains such as monomer-monomer binding site, DNAase-I-dependent actin-actin binding site and actin-severing domain, which modified the polypeptide chain exposure.