Formins play important role in Leishmania physiology by acting as cytosolic actin bundlers

Formins are a highly conserved eukaryotic family of proteins that regulate actin dynamics. They play important physiological roles in cell adhesion, motility, vesicular trafficking and cytokinesis. Although sequence analysis of Trypanosomatida genomes predicted multiple formin-encoding genes, none of them are functionally characterized yet. We report here experimental identification and functional characterization of two constitutively expressed formins from the trypanosomatid protozoa Leishmania major viz. LmForminA and LmForminB. These formins exhibited irregular cytosolic distribution that co-localized with actin patches. Co-sedimentation assay and surface plasmon resonance confirmed that purified LmForminA and B FH2 domains can bind actin, albeit with differential affinity. Interestingly, both LmForminA and B FH2 domains were found to be actin bundlers as revealed by low-speed co-sedimentation assay and TIRF microscopy. LmForminA and B also had actin-nucleating activities, which was abolished by mutating their conserved Ile residue crucial for actin assembly. The Ile-mutant formins, however, retained their actin binding and bundling properties. Treatment of Leishmania cells with formin inhibitor SMIFH2 severely perturbed parasite growth and morphology indicating that Lmformins are physiologically important and may be considered as novel drug targets.