IMPAIRMENT OF MUSCLE FUNCTION CAUSED BY MUTATIONS OF PHOSPHORYLATION SITES IN MYOSIN REGULATORY LIGHT-CHAIN

MYOSIN regulatory light chain is phosphorylated by myosin light chain kinase at conserved serine and threonine residues in a number of speci es(1). Phosphorylation of myosin regulatory light chain regulates smoo th muscle contraction, but appears to have a modulatory role in striat ed muscle contraction(2). We assessed the in vivo role of myosin regul atory light chain phosphorylation in the striated muscles of Drosophil a melanogaster by substituting alanine at each or both conserved myosi n light chain kinase-dependent phosphorylation sites, serine 66 and se rine 67. We report here that myosin light chain kinase-dependent phosp horylation is not required for myofibrillogenesis or for the developme nt of maximal isometric force in Indirect flight muscles. However, mut ants with substitutions at the major phosphorylation site (serine 66) or with the double substitutions had reduced power output in isolated flight muscle fibres and reduced flight ability, showing that myosin r egulatory light chain phosphorylation is a key determinant of the stre tch activation response in Drosophila.