Overexpression of MID2 suppresses the profilin-deficient phenotype of yeast cells

Profilin-deficient Saccharomyces cerevisiae cells show abnormal growth, actin localization, chitin deposition, bud formation and cytokinesis. Previous studies have also revealed a synthetic lethality between pfy1 and late secretory mutants, suggesting a role for profilin in intracellular transport. In this work, we document further the secretion defect associated with the pfy1Δ mutant. Electron microscopic observations reveal an accumulation of glycoproteins in the bud and in the mother cell. The MAT a, pfy1Δ cells mate as well as wild-type cells, while the mating efficiency of MAT α, pfy1Δ cells is reduced. Pulse-chase experiments demonstrate an accumulation of the 19 kDa α-factor precursor and delayed secretion of the mature α-factor. The TGN protein Kex2p is the principal enzyme responsible for the endoproteolytic cleavage of the α-factor precursor. An immunofluorescence detection of Kex2p shows an altered localization in pfy1Δ cells. Instead of a discrete punctate distribution, the enzyme is dispersed throughout the cytoplasm. A high-copy-number plasmid containing MID2 , which encodes a potential transmembrane protein involved in cell cycle control, suppresses the abnormal growth, actin distribution, α-factor maturation and the accumulation of intracellular membranous structures in pfy1Δ cells.