The stress response to loss of signal recognition particle function in Saccharomyces cerevisiae

It has been shown previously that growth and endoplasmic reticulum (ER) translocation defects occur in response to depletion of the 54-kDa subunit of signal recognition particle (SRP54) in Saccharomyces cerevisiae (Hann, B. B., and Walter, P. (1991) Cell 67, 131-144). We report here that cells depleted of SRP54p undergo a general stress response, the onset of which is observed almost two-cell doublings after SRP54 protein levels fall below the limits of detection. The stress response to SRP54p depletion occurs in two distinct phases, unlike the response to other stressors such as heat shock. In the initial phase, the cytoplasmic Hsp70 levels are drastically increased coincident with an abrupt slowing of growth and accumulation of untranslocated species of the ER-resident chaperone BiP. During this first response, levels of the yeast DnaJ homolog Ydj1p are also increased. In the second phase, which is detected 5 h later, levels of the cytoplasmic heat shock proteins Hsp82 and Hsp104 are increased. BiP is also induced during this second phase, while the ER levels of the resident foldase protein disulfide isomerase are significantly reduced. Since only those cytoplasmic stress proteins which have been shown to participate in membrane translocation are induced in the first phase, these findings indicate the presence of a stress response specific to accumulation of secretory protein precursors in the cytoplasm.