Characterization of the mRNA capping apparatus of Candida albicans

The mRNA capping apparatus of the pathogenic fungus Candida albicans consists of three components: a 520- amino acid RNA triphosphatase (CaCet1p), a 449-amino acid RNA guanylyltransferase (Cgt1p), and a 474-amino acid RNA (guanine-N7-)-methyltransferase (Ccm1p). The fungal guanylyltransferase and methyltransferase are structurally similar to their mammalian counterparts, whereas the fungal triphosphatase is mechanistically and structurally unrelated to the triphosphatase of mammals. Hence, the triphosphatase is an attractive antifungal target. Here we identify a biologically active C-terminal domain of CaCet1p from residues 202 to 520. We find that CaCet1p function in vivo requires the segment from residues 202 to 256 immediately flanking the catalytic domain from 257 to 520. Genetic suppression data implicate the essential flanking segment in the binding of CaCet1p to the fungal guanylyltransferase. Deletion analysis of the Candida guanylyltransferase demarcates an N-terminal domain, Cgt1(1-387)p, that suffices for catalytic activity in vitro and for cell growth. An even smaller domain, Cgt1(1-367)p, suffices for binding to the guanylyltransferase docking site on yeast RNA triphosphatase. Deletion analysis of the cap methyltransferase identifies a C-terminal domain, Ccm1(137-474)p, as being sufficient for cap methyltransferase function in vivo and in vitro. Ccm1(137-474)p binds in vitro to synthetic peptides comprising the phosphorylated C-terminal domain of the largest subunit of RNA polymerase II. Binding is enhanced when the C-terminal domain is phosphorylated on both Ser-2 and Ser-5 of the YSPTSPS heptad repeat. We show that the entire three-component Saccharomyces cerevisiae capping apparatus can be replaced by C. albicans enzymes. Isogenic yeast cells expressing "all-Candida" versus "all-mammalian" capping components can be used to screen for cytotoxic agents that specifically target the fungal capping enzymes.