Actin and myosin in Gregarina polymorpha.

Actin and two class XIV unconventional myosins have been cloned from Gregarina polymorpha, a large protozoan parasite inhabiting the gut of the mealworm Tenebrio molitor. These proteins were most similar to their homologues expressed in the coccidian and haemosporidian Apicomplexa such as Toxoplasma and Plasmodium despite the significant morphological differences among these parasites. Both actin and G. polymorpha myosin A (GpMyoA), a 92.6-kDa protein characterized by a canonical myosin head domain and short, highly basic tail, localized to both the longitudinally-disposed surface membrane folds (epicytic folds) of the parasite as well as to the subjacent rib-like myonemes that gird the parasite cortex. G. polymorpha myosin B (GpMyoB), a 96.3-kDa myosin, localized exclusively to the epicytic folds of the parasite. Both myosins were tightly associated with the cortical cytoskeleton and were solubilized only with a combination of high salt and detergent. Both GpMyoA and GpMyoB could bind to actin in an ATP-sensitive fashion. The distribution of actin and the unconventional myosins in G. polymorpha was consistent with their proposed participation in both the rapid (1-10 microm/sec) gliding motility exhibited by the gregarines as well as the myoneme-mediated bending motions that have been observed in these parasites.