Mutations in the Dictyostelium heterotrimeric G protein alpha subunit G alpha5 alter the kinetics of tip morphogenesis.

Tip morphogenesis during the Dictyostelium developmental life cycle is a process by which prestalk cells sort to form the anterior region of the multicellular organism. We show that the temporal regulation of this morphological process is dependent on the copy number of the Dictyostelium G alpha5 gene. Tip formation is delayed in aggregates of g alpha5 null mutant cells and accelerated in aggregates overexpressing the G alpha5 gene compared to tip formation in wild-type cells. The onset of cell-type-specific gene expression associated with mound formation and tip morphogenesis is also temporally altered in G alpha5 mutants. Tip morphogenesis in chimeric organisms of G alpha5 mutants and wild-type cells is dependent on the copy number of the G alpha5 gene, indicating that G alpha5 function plays an integral role in the intercellular signaling of this stage of development. The G alpha5 gene encodes a G alpha subunit that has 51% identity to the Dictyostelium G alpha4 subunit. Like the G alpha4 gene, the G alpha5 gene is expressed in a subset of cells distributed throughout the multicellular organism, with a distribution that is similar to the anterior-like cell population. Amino acid substitutions in the G alpha5 subunit analogous to substitutions altering guanine nucleotide binding and hydrolysis in other G alpha subunits had no apparent effect on the rate of tip formation when a single copy of the mutant gene was used to replace the wild-type gene. Overexpression of these mutant G alpha5 genes by increased gene dosage resulted in cell death, suggesting that high levels of the altered subunits have detrimental effects during vegetative growth.