Fine tuning the myosin motor: The role of the essential light chain in striated muscle myosin

It has long been known that the essential light chain isoform of striated muscle affects the function of the myosin motor. There are two isoforms: A1-type and A2-type that differ by the presence of an extra 40 amino acids at the N-terminus of A1-type light chains. Evidence has accumulated from a variety of experimental techniques that this extension of A1-type light chains makes a direct contact with actin, increasing the overall affinity between myosin and actin and that this interaction is responsible for the modulation of myosin motor function. Some recent work, however, has provided some contradictory data. Experiments using more physiologically relevant forms of myosin have suggested that the effect of the N-terminal region of A1-type light chains may, in some circumstances, be to weaken, rather than strengthen the actin-myosin interaction. Work with transgenic mice in which this region was mutated showed no measurable phenotypic effects on either muscle or whole organism function questioning the in vivo significance of the light chain-actin interaction. It is also possible that the essential light chain has other functions in the cell. There is evidence that the protein may interact with IQGAP, a regulator of the actin cytoskeleton. The consequences of this interaction are unknown. This review aims to summarise the biochemical data on striated muscle myosin essential light chain isoform function and to reconcile it with these recent discoveries.