Mutations in the Dictyostelium heterotrimeric G protein α subunit Gα5 alter the kinetics of tip morphogenesis

Tip morphogenesis during the Dictyostelium developmen-tal 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 Gα5 gene. Tip formation is delayed in aggregates of gα5 null mutant cells and accelerated in aggregates overexpressing the Gα5 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 Gα5 mutants. Tip morphogenesis in chimeric organisms of Gα5 mutants and wild-type cells is dependent on the copy number of the Gα5 gene, indicating that Gα5 function plays an integral role in the intercellular signaling of this stage of development. The Gα5 gene encodes a Gα subunit that has 51% identity to the Dic-tyostelium Gα4 subunit. Like the Gα4 gene, the Gα5 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 Gα5 subunit analogous to substitutions altering guanine nucleotide binding and hydrolysis in other Gα 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 Gα5 genes by increased gene dosage resulted in cell death, suggesting that high levels of the altered subunits have detrimental effects during vegetative growth.