An atypical F-actin capping protein modulates cytoskeleton behaviors crucial to colonization of Trichomonas vaginalis

Cytoadherence and consequential migration are crucial for pathogens to establish colonization in the host. In contrast to the nonadherent isolate of Trichomonas vaginalis, the adherent one expresses more actin-related machinery proteins with more active flagellate-amoeboid morphogenesis, amoeba migration, and cytoadherence, activities that were abrogated by an actin assembly blocker. By immunoprecipitation coupled with label-free quantitative proteomics, an F-actin capping protein (TvFACPα) was identified from the actin-centric interactome, with an atypically greater binding preference to G-actin than F-actin. TvFACPα partially colocalized with F-actin at the parasite pseudopodia protrusion and formed the protein complexes with α-actin through its c-terminal domain. Meanwhile, TvFACPα overexpression suppresses F-actin polymerization, amoeboid morphogenesis, and cytoadherence in this parasite. Ser² phosphorylation of TvFACPα enriched in the amoeboid stage of adhered trophozoites was reduced by a CKII inhibitor. The site-directed mutagenesis and CKII inhibitor treatment revealed that Ser² phosphorylation acts as a switching signal to alter TvFACPα actin-binding activity and consequent actin cytoskeleton behaviors. Through CKII signaling, TvFACPα also controls the conversion of adherent trophozoite from amoeboid migration to flagellate form with axonemal motility. Together, CKII-dependent Ser² phosphorylation regulates TvFACPα binding actin to fine-tune cytoskeleton dynamics and drive crucial behaviors underlying host colonization of T. vaginalis.