A single Drosophila melanogaster myosin light chain kinase gene produces multiple isoforms whose activities are differently regulated.

Myosin light chain kinase (MLCK) specifically phosphorylates the myosin regulatory light chain in a calcium/calmodulin (Ca2+/CaM)-dependent manner in animal cells. The roles of MLCK are not fully understood, particularly in nonmuscle cells. Therefore, we cloned and characterized a Drosophila MLCK gene as the first step in a genetic analysis of this process. Four transcripts are produced from this gene. These transcripts encode at least three isoforms (isoform-I, -II and -III), which share a kinase domain, a fibronectin type III motif and an immunoglobulin C2 motif. However, regulatory regions differ between isoform-I/II and -III due to the alternative splicing of the exon encoding a CaM-binding domain. As a result, isoform-I and -II are Ca2+/CaM-dependent forms, whereas isoform-III is a Ca2+/CaM-independent form. Northern blotting and in situ hybridization showed that the expressions of these isoforms are distinctly regulated in stage- and tissue-dependent manners. Isoform-I seems to be expressed ubiquitously, while isoform-III is expressed predominantly in muscle tissues. In contrast to these isoforms, isoform-II is specific to late pupa and adult. In Drosophila, a single MLCK gene produces multiple isoforms whose regulatory regions and expression patterns are different. These differences suggest various cellular functions of MLCK in Drosophila.