In mammalian cells RACK1 serves as a scaffold protein for numerous components of diverse signal transduction pathways, of which PKCβII is the most recognized 
. In this study we show that in the budding yeast, loss of the RACK1 ortholog gene, asc1
, results in phenotypes that characterize some PKC signaling mutants. In particular, asc1
Δ strain was sensitive to staurosporine (), a specific inhibitor of Pkc1p in yeast 
, and showed increased sensitivities to hypotonic- and heat shock conditions and to Zymolase treatment (, , and ). These are known to provoke a cell-wall stress, which requires a functional PKC signaling for cells to survive.
The dynamics of asc1
Δ loss of viability, whereby more than half of asc1
Δ cells were damaged within the first minute of exposure to hypotonic conditions, is highly similar to the response of pkc1
Δ strain (). However, unlike pkc1
Δ strain that displayed unchanged to slow reduction in viability after the initial drop in survival, asc1
Δ strain slowly resumed its growth. This may imply that pkc1
Δ sensitivity to hypotonic conditions involves two distinct processes, of which asc1
Δ susceptibility is linked only to the first, immediate one. Indeed, Levin et al.
have shown that PKC1
-deleted cells that possessed buds of any size underwent immediate lysis upon transfer to medium lacking osmotic stabilizers, while non budded cells arrested at the early stages of bud formation 
. Therefore, whereas Pkc1p function is important to all stages of bud formation Asc1p appears to have a role only in the mid-to late stages of this process.
Several observations in this work suggest that Asc1p is connected to Pkc1p by a different mechanism then in higher eukaryotes. First, in mammalian cells, RACK1 serves as a scaffold that mediates the phosphorylation and activation of the MAPK JNK by PKCβII 
. However, our data suggest that asc1
Δ cells contain high basal levels of PKC signaling (), which comes in agreement with the hyper-phsphorylation of the terminal MAPK, Slt2/Mpk1p, in asc1
Δ background 
. This observation raises the possibility that the PKC signaling is activated in asc1
Δ strain to compensate for its cell-wall sensitivity due to loss of function of other mechanism. Second, in mammalian cells PKCβII-RACK1 interaction appears to target PKCβII to distinct intracellular locations, which vary between different cell types 
. In yeast, full-length Pkc1p appears to reside predominantly at the bud-neck of medium to large sized buds and at the tip of small-sized buds, and to become re-localized to the cell's periphery upon exposure to cell-wall stress 
. Our results show that loss of Asc1p has no effect on Pkc1p localization at steady-state growth conditions nor on its re-distribution after cell-wall stress (), and has no influence on Pkc1p fractionation with membrane-compartments (). Third, while a physical interaction between Asc1 and Pkc1 proteins cannot be ruled out, we were unable to support these by two hybrid analyses or co-immunoprecipitation (data not shown). Moreover, the genetic interactions between the two genes suggest that they do not act in the same pathway. Specifically, double-knockout mutant lacking both asc1
genes displayed synergistic sensitivities to cell-wall stress conditions (). Taken together, our observations suggest that in S.cerevisiae
, Asc1p contribution to cell-wall integrity is not through the conventional Bck1-Mkk1/2-Mpk1 MAPK module that acts downstream to Pkc1p. Rather, Asc1p appears to function in parallel to Pkc1p, or to coordinate between the PKC signaling pathway and other cell-wall integrity related mechanisms, to the most.
How might Asc1p affect the integrity of the cell-wall? A likely mechanism is by regulating the translation of mRNAs that encode for cell-wall proteins. Indeed, mutations in Asc1p that were shown to reduce its ability to associate with ribosomes, resulted in also increased sensitivity to calcofluor white, a cell-wall perturbing agent 
. Based on the immediate lysis of asc1
Δ cells upon hypotonic shock conditions () that was independent of de-novo synthesis of proteins (), we suggest that regulation of translation by Asc1 is exerted on bud-site related mRNAs during steady-state growth, rather than in response to cell-wall stress.