Fil1, a G-protein alpha-subunit that acts upstream of cAMP and is essential for dimorphic switching in haploid cells of Ustilago hordei

A constitutive mutation, fill, that causes filamentous growth in the haplophase of the dimorphic smut fungus Ustilago hordei, was previously shown to be genetically associated with a 50-kb deletion within a 940-kb chromosome. Physiological studies suggested that a gene that functions upstream of adenylyl cyclase was deleted in the mutant. Representational difference analysis of isolated chromosomes was used to obtain deletion-specific DNA probes and corresponding genomic cosmid clones. Complementation analysis identified a cosmid clone and subsequently a 2.1-kb insert that converted transformants of the mutant strain10.1a(fill) from the filamentous to the sporidial cell type. A single open reading frame of 354 codons that encodes a putative a-subunit of the heterotrimeric G-proteins was identified. Fill displayed a high degree of sequence identity to Gpa1 from the basidiomycete Cryptococcus neoformans and CPG-2 from the ascomycete Cryphonectria parasitica. FIL1, when introduced on a self-replicating vector, was found to suppress filamentous growth of starved haploid wild-type strains and restore normal mating response to the fill mutant, but did not suppress sexual dimorphism of either strain. Fill appears to function analogously to mammalian G alpha proteins, which are coupled to cAMP production via adenylyl cyclase, to regulate dimorphic switching in U. hordei.