Evolution of the regulatory control of vertebrate striated muscle: the roles of troponin I and myosin binding protein-C

Abstract

Troponin I (TnI) and myosin binding protein-C (MyBP-C) are key regulatory proteins of contractile function in vertebrate muscle. TnI modulates the Ca(2+) activation signal, while MyBP-C regulates cross-bridge cycling kinetics. In vertebrates, each protein is distributed as tissue-specific paralogs in fast skeletal (fs), slow skeletal (ss), and cardiac (c) muscles. The purpose of this study is to characterize how TnI and MyBP-C have changed during the evolution of vertebrate striated muscle and how tissue-specific paralogs have adapted to different physiological conditions. To accomplish this we have completed phylogenetic analyses using the amino acid sequences of all known TnI and MyBP-C isoforms. This includes 99 TnI sequences (fs, ss, and c) from 51 different species and 62 MyBP-C sequences from 26 species, with representatives from each vertebrate group. Results indicate that the role of protein kinase A (PKA) and protein kinase C (PKC) in regulating contractile function has changed during the evolution of vertebrate striated muscle. This is reflected in an increased number of phosphorylatable sites in cTnI and cMyBP-C in endothermic vertebrates and the loss of two PKC sites in fsTnI in a common ancestor of mammals, birds, and reptiles. In addition, we find that His(132), Val(134), and Asn(141) in human ssTnI, previously identified as enabling contractile function during cellular acidosis, are present in all vertebrate cTnI isoforms except those from monotremes, marsupials, and eutherian mammals. This suggests that the replacement of these residues with alternative residues coincides with the evolution of endothermy in the mammalian lineage.

The influence of trout cardiac troponin I and PKA phosphorylation on the Ca2+ affinity of the cardiac troponin complex

“…A similar scheme for the evolution of other contractile proteins has also been suggested by Oota and Saitou (Ota and Saitou, 1999). We have also suggested that the similarity between cTnI from fish species and ssTnI indicates that there has been little change in the sequence of fish cTnI since this group diverged from the vertebrate lineage (Shaffer and Gillis, 2010). The results of the current study demonstrating that trout cTnI increases the Ca 2+ affinity of the cTn complex, as does ssTnI, support this hypothesis.…”

“…These results provide evidence that cTnI from fish can be viewed as ancestral or primitive forms of the paralogous protein in mammalian cardiac tissue. This hypothesis is supported by previous phylogenetic studies of vertebrate TnI (Shaffer and Gillis, 2010). The sensitizing effect of trout cTnI on the mammalian cTn complex suggests that this protein is involved in the comparatively high Ca 2+ sensitivity of the trout heart and therefore helps enable cardiac function at low temperatures.…”

“…Following a phylogenetic study of all known TnI isoforms, we have recently proposed that the three muscle-specific isoforms of TnI (cardiac, ss and fs) were produced via two genome duplications that occurred in an ancestor of all extant vertebrate groups (Shaffer and Gillis, 2010). Our analysis indicated that a fsTnI 'like' paralog is the ancestral form of the protein, which was then replicated to produce two isoforms via genome duplication.…”

“…Interestingly, trout cTnI lacks the cardiac-specific N-terminal extension that contains the PKA/PKC/PKD phosphorylation targets (serine residues) at positions 23 and 24 (Fig.2). All other cTnIs cloned from fish species also lack this extension (Shaffer and Gillis, 2010). Trout cTnI does, however, contain Ser residues at positions 14 and 16.…”

“…The limited response to PKA phosphorylation of the cTn complex containing trout cTnI is due to the lack of the N-terminal extension in the protein. The N-terminal extension present in cTnI from all other vertebrate groups allows for greater regulation of cTn function and it evolved after fish diverged from the vertebrate lineage (Shaffer and Gillis, 2010). The lack of this protein domain in cTnI therefore limits the ability of the animal to regulate cardiac contractility.…”