Slt2, the MAPK of the cell wall integrity (CWI) pathway, connects different signaling pathways and performs different functions in the protective response of S. cerevisiae to stress. Previous work has evidenced the relation of the CWI pathway and the unfolded protein response (UPR), a transcriptional program activated upon endoplasmic reticulum (ER) stress. However, the mechanisms of crosstalk between these pathways and the targets regulated by Slt2 under ER stress remain unclear. Here, we demonstrated that ectopic expression of GFA1, the gene encoding the first enzyme in the synthesis of UDP-GlcNAc by the hexosamine biosynthetic pathway (HBP) or supplementation of the growth medium with glucosamine (GlcN), increases the tolerance of slt2 mutant cells to different ER-stress inducers. Remarkably, GlcN also alleviates the sensitivity phenotype of cells lacking IRE1 or HAC1, the main actors in controlling the UPR. The exogenous addition of GlcN reduced the abundance of glycosylated proteins and triggered autophagy. We also found that TORC1, the central stress and growth controller, is inhibited by tunicamycin exposure in cells of the wild-type strain but not in those lacking Slt2. Consistent with this, the tunicamycin-induced activation of autophagy and the increased synthesis of ATP in response to ER stress were absent by knock-out of SLT2. Altogether, our data placed Slt2 as an essential actor of the ER stress response by regulating the HBP activity and the TORC1-dependent signaling.

Summary l‐rhamnose is found in nature mainly as a component of structural plant polysaccharides and can be used as a carbon source by certain microorganisms. Catabolism of this sugar in bacteria, archaea and fungi occurs by two routes involving either phosphorylated or non‐phosphorylated intermediates. Unlike the corresponding pathway in yeasts, the metabolic details of the non‐phosphorylated pathway in filamentous fungi are not fully defined. The first three genes (lraA, lraB and lraC) of the non‐phosphorylated pathway in Aspergillus nidulans have recently been studied revealing dependence on lraA function for growth on l‐rhamnose and α‐l‐rhamnosidase production. In the present work, two genes encoding the subsequent steps catalysed by l‐2‐keto‐3‐deoxyrhamnonate (l‐KDR) aldolase (AN9425) and l‐lactaldehyde dehydrogenase (AN0554) are identified. Loss‐of‐function mutations cause adverse growth effects on l‐rhamnose. Akin to genes lraA‐C and those encoding rhamnosidases (rhaA, rhaE), their expression is induced on l‐rhamnose via the transcriptional activator RhaR. Interestingly, the aldolase belongs to the ftablamily of bacterial l‐KDR aldolases (PF03328/COG3836) and not that of yeasts (PF00701/COG0329). In addition, AN0554 corresponds to the previously characterized aldA gene (encodes aldehyde dehydrogenase involved in ethanol utilization) thus revealing a previously unknown role for this gene in the catabolism of l‐rhamnose.