Hos2p/Set3p deacetylase complex signals secretory stress through the Mpk1p cell integrity pathway.

The budding yeast Hos2p protein is a member of the histone deacetylase (HDAC) family related to the founding class I HDAC Rpd3p (30, 37, 38, 40, 44). Biochemical purification has revealed that Hos2p is a component of the multisubunit Set3p complex (SET3C), which includes the SET domain-containing protein Set3p and several uncharacterized components, including Sif2p, Snt1p, and Yil112w (37). Further biochemical analysis indicated that Hos2p, Set3p, Sif2p, and Snt1p form the core of this complex, while three additional proteins, Hst1p, Sum1p, and Cpr1p, appear to be more peripherally associated (37). A functional role for SET3C was shown in the transcriptional repression of the early/middle class of sporulation-specific genes including the key meiotic regulators IME2 and NDT80 (37). In addition, genome-wide chromatin immunoprecipitation (ChIP) analysis has revealed that Hos2p is associated with genes that are actively transcribed, including those that encode small and large ribosomal subunits (44). It is interesting that ribosome biogenesis and meiotic induction are regulated by nutritional signals (18, 28). These studies suggest that the Hos2p complex might play a role in the responses to specific environmental stimuli. However, any role for the Hos2p complex in a signaling capacity has not been elucidated. Maintaining a functional secretory pathway is critical for cell survival. Eukaryotic cells have evolved multiple means to respond to perturbations in the secretory pathway. One such signaling pathway feeds into the endoplasmic reticulum (ER), which ensures that secretory proteins are properly folded and modified prior to ER exit and entry into the secretory pathway (36, 39). Defects in this pathway lead to the toxic accumulation of misfolded proteins in the ER and the activation of a survival response known as the unfolded protein response (UPR) (7, 29, 36, 41). This occurs through the activation of an ER-based RNase/kinase called Ire1p, which catalyzes the nonconventional splicing of the HAC1 transcript, a bZIP transcription factor (7). Upon proper splicing, Hac1p is translated, enters the nucleus, and activates UPR genes to appropriately respond to the accumulating ER stress. Interestingly, one major class of genes induced by the UPR are those involved in the secretory pathway (42). Furthermore, inhibition of secretory function by brefeldin A or secretory pathway-deficient mutants (sec− mutants), such as temperature-sensitive alleles of both COPI and COPII transport proteins, activates the UPR (2, 31, 32). These observations suggest that the UPR is intimately linked to the secretory pathway. In addition to UPR activation, secretory stress represses the machinery required for ribosomal biogenesis and protein synthesis (24, 25, 28, 33). By coupling the secretory status to UPR activation and protein synthesis pathways, cells may quickly and efficiently monitor the secretory network and establish the proper balance between cell growth and protein production. It was proposed that perturbations in the secretory pathway may be sensed ultimately at the cell membrane through deficiencies in membrane lipids and/or essential membrane proteins, leading to Pkc1p activation (33). How Pkc1p relays the secretory stress signal to elicit a stress response remains unknown. However, previous studies indicated that the secretory stressor tunicamycin activates the yeast mitogen-activated protein (MAP) kinase Mpk1p (1, 4), a critical component of the yeast cell integrity pathway, which is a linear kinase cascade comprising Pkc1p → Bck1p (MEK kinase) → Mkk1/2p (MEK) → Mpk1p/Slt2p (MAP kinase) (22, 23, 35, 47). The Mpk1p cell integrity pathway is activated in response to a variety of environmental stimuli, including heat shock, cell wall stress, actin depolymerization, hypo-osmotic stress, and mating pheromone (10). Mpk1p phosphorylates and activates downstream transcriptional effectors including Rlm1p (15, 16, 45, 46) and triggers the destruction of the transcriptional repressor cyclin C (Ssn8p/Ume3p/Srb11p) (5, 6). Whether the Mpk1p cell integrity pathway regulates the response to secretory stress remains unknown. Here, we provide evidence that the deacetylase Hos2p and its associated complex are required for an efficient response to secretory stress. This response requires a core set of proteins within SET3C including Hos2p, Set3p, Sif2p, and Snt1p. Unexpectedly, we demonstrate that two branches of secretory stress signaling, the UPR and the ribosomal repression response, remain intact in the absence of SET3C function. In addition, we have discovered that the Hos2p-dependent stress signal is transduced via the Mpk1p MAP kinase cascade leading to activation of Rlm1p-dependent transcription. Our results identify the Hos2p/Set3p complex (SET3C) as a novel component critical for cell survival in response to secretory stress.