Comparison of the Activities of Two Putative Formins from Leishmania major

Formins are actin-dynamics-regulating proteins that are required for several actin-based cellular processes such as cell motility and cytokinesis. While much progress has been made in the study of mammalian formin’s biochemical properties and cell biology, the functional evolution of formins remains less well understood. Two homologs of this protein have recently been identified in the protozoan parasite Leishmania major; Formin A and Formin B. As a dynamic actin cytoskeleton is required for such parasites for its survival and effective pathogenesis, the functions of such actin regulatory proteins are extremely crucial. We tried to biochemically characterize the FH2 domains of these putative formins which might shed light in understanding the evolution of formins. Results of actin co-sedimentation assays indicated that FH2 domain of both formin A and formin B can bind actin filaments but cannot bundle them. Kinetic measurements showed that, like conventional formins, Leishmania formin A can act as a potent actin nucleator. However, Formin B drastically decreased the rate of actin polymerization and is suspected to cap the actin barbed ends, inhibiting the addition of monomers to that end. Further investigation is required to establish these observations as some aspects of their interactions with actin are still not completely understood.