The SR protein kinase Sky2 is involved in regulation of dipeptide transport in Candida albicans

Metabolic adaption to different host niches is one of the most important virulence traits of human fungal pathogens. The metabolic versatility of fungi and other cellular processes such as proliferation, morphogenesis and stress responses are tightly regulated by complex networks in which protein kinases play a crucial role. Serine-arginine (SR) protein kinases are highly conserved in all eukaryotes, but their function in pathogenic Candida spp. has not yet been investigated. C. albicans genome encodes two (Sky1, Sky2) and Candida glabrata one (Sky1) homolog of the human SR protein kinase 1 (SRPK1). We utilized deletion strains of the respective genes in both fungi in order to examine their cellular functions. C. glabrata and C. albicans strains lacking SKY1 exhibited higher resistance to osmotic stress and toxic polyamine concentrations to their ortholog in the model yeast Saccharomyces cerevisiae Sky1. Deletion of SKY2 in C. albicans resulted in impaired utilization of dipeptides as the sole nitrogen source. Subsequent phosphoproteomic analysis identified Ptr22, a transporter of di- and tri-peptides in C. albicans, as a potential Sky2 substrate. Overexpression of PTR22 in the sky2∆ restored the ability to grow on dipeptides as the sole nitrogen source and rendered the cells more susceptible to the dipeptide antibiotics Polyoxin D and Nikkomycin Z. Altogether, our results demonstrate that the two SR-like protein kinases in C. albicans have divergent functions: C. albicans and C. glabrata Sky1 is functionally similar to Sky1 in S. cerevisiae, whereas Sky2 regulates uptake of dipeptides.