The distinct biochemical property enables thymidylate kinase as a drug target and participates in pyrimidine drug sensitivity in Candida albicans

The ability to overcome drug resistance in outbreaks of Candida albicans infection is an unmet need in health management. Here, we investigated CDC8, which encodes thymidylate kinase (TMPK), as a potential drug target for the treatment of C. albicans infection. In this study, we found that the specific region spanning amino acids 106-123, namely, the Ca-loop of C. albicans TMPK (CaTMPK) contributes to the hyperactivity of this enzyme compared to the human enzyme (hTMPK) and to the utilization of deoxyuridine monophosphate (dUMP)/ deoxy-5-Fluorouridine monophosphate (5-FdUMP) as a substrate. Notably, CaTMPK but not hTMPK enables dUTP/5-FdUTP-mediated DNA toxicity in yeast. CRISPR-mediated deletion of this Ca-loop in C. albicans demonstrated the critical role of this Ca-loop in fungal growth and susceptibility to 5-Fluorouridine (5-FUrd). Moreover, pathogenic and drug-resistant C. albicans clones were similarly sensitive to 5-FUrd. Thus, this study not only identified a target site for the development of CaTMPK-selective drugs but also revealed 5-FUrd to be a potential drug for the treatment of C. albicans infection.\n\nAuthor summaryThe emergence of drug-resistant C. albicans strains is a serious medical concern that may be addressed by targeting an essential fungal enzyme. CDC8 encodes thymidylate kinase (TMPK), which is the key enzyme required for dTTP synthesis and is an essential gene for yeast growth. Therefore, the differences of TMPK between human and C. albicans can be a potential drug targeting site. This study defines a specific Ca-loop unique to CaTMPK from C. albicans, contributing to hyper-activity over human enzyme (hTMPK). CRSPR-edited deletion of this loop also suppressed the growth of C. albicans. Moreover, we present evidence that this loop enables dUMP utilization by CaTMPK, but not hTMPK. CaTMPK is also capable of using 5-FdUMP as a substrate, which contributes to 5-FUrd-mediated toxicity. Importantly, we found that many drug resistant pathogenic C. albicans isolates from patients are sensitive to 5-FUrd, which has not been used as a drug against fungal infection.