Molecular evolution of immunoglobulin and fibronectin domains in titin and related muscle proteins

The family of regulatory and structural muscle proteins, which includes the giant kinases titin, twitchin and projection, has sequences composed predominantly of serially linked immunoglobulin I set (Ig) and fibronectin type III (FN3) domains. This paper explores the evolutionary relationships between 16 members of this family. In titin, groups of Ig and FN3 domains are arranged in a regularly repeating pattern of seven and 11 domains. The 11- domain super-repeat has its origins in the seven-domain super-repeat and a model for the duplications which gave rise to this super-repeat is proposed. A super-repeat composed solely of immunoglobulin domains is found in the skeletal muscle isoform of titin. Twitchin and projection, which are presumed to be orthologs, have undergone significant insertion/deletion of domains since their divergence. The common ancestry of myomesin, skelemin and M- protein is shown. The relationship between myosin binding proteins (MyBPs) C and H is confirmed, and MyBP-H is proposed to have given rise to MyBP-C by the acquisition of some titin domains.