Generation of an isogenic collection of yeast actin mutants and identification of three interrelated phenotypes

A large collection of yeast actin mutations has been previously isolated an d used in numerous studies of actin cytoskeletal function. However, the var ious mutations have been in congenic, rather than isogenic, backgrounds, ma king it difficult to compare the subtle phenotypes that are characteristic of these mutants. We have therefore placed 27 mutations in an isogenic back ground. We used a subset of these mutants to compare the degree to which di fferent actin alleles are defective in sporulation, endocytosis, and growth on NaCl-containing media. We found that the three phenotypes are highly co rrelated. The correlations are specific and not merely a reflection of gene ral growth defects, because the phenotypes are not correlated with growth r ates under normal conditions. Significantly, those actin mutants exhibiting the most severe phenotypes in all three processes have altered residues th at cluster to a small region of the actin crystal structure previously defi ned as the fimbrin (Sac6p)-binding site. We examined the relationship betwe en endocytosis and growth on salt and found that shifting wild-type or acti n mutant cells to high salt reduces the rate of or-factor internalization. These results suggest that actin mutants may be unable to grow on salt beca use of additive endocytic defects (due to mutation and salt).