Preserved close linkage between the genes encoding troponin I and troponin T, reflecting an evolution of adapter proteins coupling the Ca²⁺ signaling of contractility

Ca²⁺-regulated motility is essential to numerous cellular functions, including muscle contraction. Systems with troponin C, myosin light chain, or calmodulin as the Ca²⁺ receptor have evolved in striated muscle and other types of cells to transduce the cytoplasm Ca²⁺ signals into allosteric conformational changes of contractile proteins. While these Ca²⁺ receptors are homologous proteins, their coupling to the responding elements is quite different in various cell types. The Ca²⁺ regulatory system in vertebrate striated muscle represents a highly specialized such signal transduction pathway consisting of the troponin complex and tropomyosin associated with the actin filament. To understand the molecular mechanism in the Ca²⁺ regulation of muscle contraction and cell motility, we have revealed a preserved ancestral close linkage between the genes encoding two of the troponin subunits, troponin I and troponin T, in the genome of mouse. The data suggest that the troponin I and troponin T genes may have originated from a single locus and evolved in parallel to encode a striated muscle-specific adapter to couple the Ca²⁺ receptor, troponin C, to the actin-myosin contractile machinery. This hypothesis views the three troponin subunits as two structure-function domains: the Ca²⁺ receptor and the signal transducing adapter. This model may help to further our understanding of the Ca²⁺ regulation of muscle contraction and the structure-function relationship of other potential adapter proteins which are converged to constitute the Ca²⁺ signal transduction pathways governing non-muscle cell motility.