Muscle function depends upon the molecular interaction of myosin and actin. This interaction and the function of each molecule are tightly regulated and have been extensively studied. In Drosophila , the indirect flight musculature (IFM) is a powerful model to study muscle structure and function as these muscles are dispensable for life under laboratory conditions. Furthermore, disruption of these muscles leads to a flightless behavior.
Flies with mutations in the muscle regulatory light chain (MLC2) that cannot be phosphorylated at the conserved Myosin light chain kinase (Mlck) target sites are flightless, but the IFM is normal. Flight impairment is due to an altered stretch activation response, thus phosphorylation of MLC2 at the Mlck target sites is important for flight.
In Drosophila , the Stretchin-Mlck (Strn-Mlck) gene encodes several Mlck-like isoforms with kinase activity as well as other isoforms lacking this domain. This work has shown that the gene is expressed in both muscle and nonmuscle cells and that some isoforms show tissue specific expression patterns.
In order to understand what role Strn-Mlck plays in MLC2 regulation, mutants were isolated. Three new mutants were identified and were shown to be new alleles of the previously identified mutant curved . Strikingly, Strn-Mlck mutants lacking kinase activity are viable. However, the mutants show a recessive flightless phenotype and defects in wing position. Electron micrographs demonstrated the flight musculature is intact, therefore, the flightless phenotype is most likely due to the wing position defect.
As mentioned, Strn-Mlck is expressed in nonmuscle cells. The myosin nonmuscle regulatory light chain is encoded by spaghetti squash (sqh) . Mutations in nonmuscle RLC at the conserved Mlck target sites are lethal. Finding that mutants lacking Strn-Mlck kinase activity are viable suggests other Mlcks exist in flies and function redundantly. Preliminary genetic studies have linked Strn-Mlck with another Mlck, CG1776 (also known as Mlck-2 ). This Mlck-like kinase has been implicated in morphogenesis in nonmuscle tissue culture cells. In the future, it will be important to analyze these two Mlcks in depth at the genetic level. Double mutants may reveal functions for these enzymes that are masked in the single mutants analyzed to date.