A block of endocytosis of the yeast cell wall integrity sensors Wsc1 and Wsc2 results in reduced fitness in vivo.

The response to cell surface stress in yeast is mediated by a set of five plasma membrane sensors. We here address the relation of intracellular localization of the sensors Wsc1, Wsc2, and Mid2 to their turnover and signaling function. Growth competition experiments indicate that Wsc2 plays an important role in addition to Wsc1 and Mid2. The two Wsc sensors appear at the bud neck during cytokinesis and employ different routes of endocytosis, which govern their turnover. Whereas Wsc1 uses a clathrin-dependent NPFDD signal, Wsc2 relies on a specific lysine residue (K495). In end3 and doa4 endocytosis mutants, both sensors accumulate at the plasma membrane, and a hypersensitivity to cell wall-specific drugs and to treatment with zymolyase is observed. A haploid strain in which endocytosis of the two sensors is specifically blocked displays a reduced fitness in growth competition experiments. If the Mid2 sensor is mobilized by the addition of an endocytosis signal, it mimics the dynamic distribution of the Wsc sensors, but is unable to complement the specific growth defects of a wsc1 deletion. These data suggest that sensor distribution is not the major determinant for its specificity.