The Tlo Proteins Are Stoichiometric Components of Candida albicans Mediator Anchored via the Med3 Subunit

The amplification of the TLO (for telomere-associated) genes in Candida albicans, compared to its less pathogenic, close relative Candida dubliniensis, suggests a role in virulence. Little, however, is known about the function of the Tlo proteins. We have purified the Mediator coactivator complex from C. albicans (caMediator) and found that Tlo proteins are a stoichiometric component of caMediator. Many members of the Tlo family are expressed, and each is a unique member of caMediator. Protein expression analysis of individual Tlo proteins, as well as the purification of tagged Tlo proteins, demonstrate that there is a large free population of Tlo proteins in addition to the Mediator-associated population. Coexpression and copurification of Tlo alpha 12 and caMed3 in Escherichia coli established a direct physical interaction between the two proteins. We have also made a C. albicans med3 Delta/Delta strain and purified an intact Mediator from this strain. The analysis of the composition of the med3 Delta Mediator shows that it lacks a Tlo subunit. Regarding Mediator function, the med3 Delta/Delta strain serves as a substitute for the difficult-to-make tlo Delta/Delta C. albicans strain. A potential role of the TLO and MED3 genes in virulence is supported by the inability of the med3 Delta/Delta strain to form normal germ tubes. This study of caMediator structure provides initial clues to the mechanism of action of the Tlo genes and a platform for further mechanistic studies of caMediator's involvement in gene regulatory patterns that underlie pathogenesis.