Abstract
Fungal tRNA ligase (Trl1) rectifies RNA breaks with 2',3'-cyclic-PO4 and 5'-OH termini. Trl1 consists of three catalytic modules: an N-terminal ligase (LIG) domain; a central polynucleotide kinase (KIN) domain; and a C-terminal cyclic phosphodiesterase (CPD) domain. Trl1 enzymes found in all human fungal pathogens are untapped targets for antifungal drug discovery. Here we report a 1.9 Å crystal structure of Trl1 KIN-CPD from the pathogenic fungus Candida albicans, which adopts an extended conformation in which separate KIN and CPD domains are connected by an unstructured linker. CPD belongs to the 2H phosphotransferase superfamily by dint of its conserved central concave β sheet and interactions of its dual HxT motif histidines and threonines with phosphate in the active site. Additional active site motifs conserved among the fungal CPD clade of 2H enzymes are identified. We present structures of the Candida Trl1 KIN domain at 1.5 to 2.0 Å resolution-as apoenzyme and in complexes with GTP•Mg2+, IDP•PO4, and dGDP•PO4-that highlight conformational switches in the G-loop (which recognizes the guanine base) and lid-loop (poised over the nucleotide phosphates) that accompany nucleotide binding.