Function of the N terminus of the myosin essential light chain of vertebrate striated muscle.

All but one (LC3-f; a fast skeletal muscle isoform) of the essential light chain isoforms of myosin (ELC) that are expressed in vertebrate striated muscles have an extended N terminus that is found neither in invertebrate ELCs nor in the majority of vertebrate smooth and nonmuscle myosin ELCs. Studies with permeabilized skeletal muscle fibers and in vitro motility assays have demonstrated that the presence of the ELC isoform lacking the N-terminal extension (LC3-f) is correlated with an increased maximal velocity of filament sliding. To examine further this modulatory role of the ELCs, a procedure was developed for the exchange of ELCs that is based on a technique for the removal of regulatory light chains from permeabilized muscle fibers. Different isoforms of the ELCs and mutant ELCs were exchanged into permeabilized skeletal muscle fibers from rabbit psoas muscle. The role of the ELCs of myosin in altering the shortening Vmax of striated muscle was confirmed. Additionally, experiments with mutant ELCs in which lysines at the extreme N terminus were replaced with alanines, demonstrated an increased shortening Vmax that coincided with removal of the positive charges contributed by the lysines. This suggests that charge interactions (i.e., salt bridges) between the N terminus of the ELC and negatively charged amino acids on the surface of actin cause a slowing of filament sliding. Whether this role in altering shortening velocity is the primary function of the extended N terminus of the ELC or whether it is merely a consequence of providing a tether between the thick and thin filaments is discussed.