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 ∼4 s^-1 in the dephosphorylated state 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 (∼ 0.7 s^-1) with the ADP release (∼ 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.