| To provide the power for flight, Drosophila flight muscles contract rapidly in an oscillatory fashion. This process depends on phosphorylation of the myosin regulatory light chain (MLC2) at the target sites for myosin light chain kinase (MLCK). In order to understand more about the mechanism, and to test the hypothesis that loss of MLCK activity impairs flight, I have undertaken an analysis of Mlck genes in Drosophila .; There are two Mlck-like genes, Strn-Mlck and Mlck-2 in Drosophila. Strn-Mlck is large and has a complex structure. The gene encodes isoforms that either contain or lack a regulatory domain. Kinase activity is regulated through binding of Ca2+/CaM to the regulatory domain, and therefore, protein isoforms lacking this domain are either constitutively active or regulated by another mechanism. Both isoforms are expressed in nonmuscle and muscle cells suggesting a role for the kinase in both tissue types. It is known that lack of phosphorylation of nonmuscle myosin regulatory light chain is lethal in Drosophila. However, Strn-Mlck mutants are viable suggesting a non-critical or redundant function for the gene in nonmuscle cells. The mutants are flightless but the pattern of MLC2 phosphorylation in the flight muscle appeared normal suggesting that STRN-MLCK is also not involved, or has a redundant function, in muscle tissue. The flightless phenotype likely results from a wing position defect because the mechanical properties of the mutant muscles are normal whereas those of MLC2 mutants in which the phosphorylation sites have been eliminated are severely impaired.; The phenotype of Strn-Mlck mutants suggested that additional MLCKs exist in Drosophila. I characterized a novel gene called Mlck-2. MLCK-2 shows a high degree of homology to the catalytic domains of MLCKs in other organisms but the protein lacks Ca2+/CaM binding sequences and may be regulated by phosphorylation. Homozygous Mlck-21 mutants are lethal, consistent with a role in phosphorylating the nonmuscle myosin regulatory light chain. Both transcripts and proteins are also found in muscles indicating a potential role for the kinase in muscle cells as well. |
