Novel myosins in the nematode, Caenorhabditis elegans

In recent years the bewildering diversity of myosin types and function has become apparent. Over ten different classes have been identified in a wide variety of organisms - ranging from protozoans to vertebrates and even higher plants. The intriguing question is - what do they all do? C. elegans, "the worm", is a relatively simple organism, both anatomically and genetically and has been studied extensively with respect to development, cell lineage and genetics. However only myosins of the conventional type (muscle or "class II" myosins) had been identified prior to the work described here. The first novel myosin from C. elegans has been cloned and fully sequenced. The predicted amino-acid sequence shows that this myosin contains the conserved motor or "head" domain responsible for actin-activated ATPase activity. This is followed by two motifs thought to be capable of interaction with members of the calmodulin class of Ca²⁺ binding proteins and a tail with a general positive charge. The sequence of the entire protein, along with expression and immunolocalisation pattern, suggests that it may be the nematode homologue of a recently discovered unconventional myosin from the rat, myr4. A multiple alignment of the 86 available conserved motor domain sequences has enabled the construction of an unrooted phylogenetic tree, which indicates that the myosins fall into 13 classes. C. elegans Myosin IA is a member of the class I myosins. The conservation of residues within this alignment has been explored further by scrutinising their position within the available myosin crystal structures. From this important residues involved in myosin function can be identified. Furthermore, the alignment allows the positioning of residues from myosins, whose structure is unknown, within the framework provided by the existing crystal structures - enabling, for example, the interpretation of existing mutations in unconventional myosins.