Characterization of Candida albicans myosin-1 motor function and its regulation by phosphorylation at TEDS site

Class I myosin is a single-headed myosin, distributed from fungi to mammals. The pathogenic yeast Candida albicans expresses a solo class I myosin gene (CaMyo1). The yeast-to-hyphae transition of Candida albicans depends on CaMyo1 and phosphorylation at its TEDS site (S366). In this study, we characterized the motor activity of CaMyo1 using purified CaMyo1(IQ2) (a truncated CaMyo1 containing the motor domain and two IQ motifs) recombinantly expressed in insect Sf9 cells. We found that S366 phosphorylation activated the actin-activated ATPase activity of CaMyo1(IQ2) from similar to 4 s(-1) in the dephosphorylated state to similar to 10 s(-1) in the phosphorylated state. The ADP release rate of acto-CaMyo1(IQ2) is >150 s(-1) regardless of S366 phosphorylation, which is at least 10 times faster than the actin-activated ATPase rate (4-10 s(-1)), suggesting that CaMyo1 is a low-duty ratio motor. Interestingly, in the absence of actin, CaMyo1(IQ2) has relatively high ATPase activity (similar to 0.7 s(-1)) with the ADP release (similar to 1 s(-1)) as the rate-limiting step, suggesting a substantial portion of cycling through futile actin-detached path. Both S366D and S366E mutants exhibit intermediate actin-activated ATPase activity between unphosphorylated and phosphorylated wild-type, demonstrating partial phosphomimetic functionality at residue S366.