b Candida albicans is a major fungal pathogen whose virulence is associated with its ability to transition from a budding yeast form to invasive hyphal filaments. The kinesin-14 family member CaKar3 is required for transition between these morphological states, as well as for mitotic progression and karyogamy. While kinesin-14 proteins are ubiquitous, CaKar3 homologs in hemiascomycete fungi are unique because they form heterodimers with noncatalytic kinesin-like proteins. Thus, CaKar3-based motors may represent a novel antifungal drug target. We have identified and examined the roles of a kinesin-like regulator of CaKar3. We show that orf19.306 (dubbed CaCIK1) encodes a protein that forms a heterodimer with CaKar3, localizes CaKar3 to spindle pole bodies, and can bind microtubules and influence CaKar3 mechanochemistry despite lacking an ATPase activity of its own. Similar to CaKar3 depletion, loss of CaCik1 results in cell cycle arrest, filamentation defects, and an inability to undergo karyogamy. Furthermore, an examination of the spindle structure in cells lacking either of these proteins shows that a large proportion have a monopolar spindle or two dissociated half-spindles, a phenotype unique to the C. albicans kinesin-14 homolog. These findings provide new insights into mitotic spindle structure and kinesin motor function in C. albicans and identify a potentially vulnerable target for antifungal drug development.
Candida albicans is a common commensal fungus that typically causes no harm to the host (1). However, in immunocompromised individuals, neonates, and patients under intensive care, it can cause serious skin and mucosal infections as well as life-threatening systemic infections (1, 2). It is also able to form tenacious biofilms on implanted medical devices (3). A major factor facilitating C. albicans virulence is its ability to readily switch between growth as a yeast, pseudohyphae, and hyphae (1, 4). In the yeast mode, daughter cells bud off and dissociate from the mother cell, while pseudohyphal cells remain connected at constricted septation sites (4). Hyphae are the invasive form and are important for virulence during systemic infections as well as for biofilm formation (1,5,6). In all of these morphological states, microtubuleassociated motor proteins play major roles in microtubule cytoskeleton remodeling, nuclear movements, chromosome segregation, and cargo transport (7-9). This provides a rationale for their use as novel targets for antifungal drugs (10).C. albicans Kar3 (CaKar3) is a kinesin-14 family member that has been shown to function in mitotic division and is critical for hypha formation and nuclear fusion during mating (9, 11). Its homolog in Saccharomyces cerevisiae has similar functions in mitosis and mating that are enabled by interactions with two kinesinlike proteins: S. cerevisiae Cik1 (ScCik1) and ScVik1 (12)(13)(14)(15)(16)(17)(18)(19)(20). ScKar3 forms complexes with each of these proteins via a central coiled-coil domain to form parallel heterodimers that are structurally ...
