Phe 356 in the yeast Ca 2+ channel component Mid1 is a key residue for viability after exposure to α-factor

Abstract

The yeast Mid1 protein is an integral membrane protein required for the viability of differentiated cells and Ca 2+ influx induced by mating pheromone. Our previous study has identified a loss-of-function mutation, F356S. The F356S mutant is completely unable to maintain viability, but still has Ca 2+ accumulation activity near the wild-type level. Here we further examined in detail the F356S mutation to unravel the function of Phe 356. After exposure to the pheromone, the F356S mutant was not fully rescued by high extracellular Ca 2+, like the mid1 null mutant, suggesting that Phe 356 and Mid1 itself are also required for viability maintenance mechanism that does not involve Ca 2+ signalling. Substitutions of hydrophilic amino acids for Phe 356 caused lethality and low Ca 2+ accumulation, but those of hydrophobic amino acids did not. Substitutions of small amino acids for Phe 356 caused a significantly reduced viability, but did not affect Ca 2+ accumulation. We suggest that the hydrophobicity of the Phe 356 residue is important for both viability maintenance and Ca 2+ uptake, and that its size for viability maintenance.