Calcium ions, present inside all eukaryotic cells, are important second messengers in the transduction of biological signals. In mammalian cells, the release of Ca2+ from intracellular compartments is required for signaling and involves the regulated opening of ryanodine and inositol-1,4,5-trisphosphate (IP3) receptors. However, in budding yeast, no signaling pathway has been shown to involve Ca2+ release from internal stores, and no homologues of ryanodine or IP3 receptors exist in the genome. Here we show that hyperosmotic shock provokes a transient increase in cytosolic Ca2+ in vivo. Vacuolar Ca2+, which is the major intracellular Ca2+ store in yeast, is required for this response, whereas extracellular Ca2+ is not. We aimed to identify the channel responsible for this regulated vacuolar Ca2+ release. Here we report that Yvc1p, a vacuolar membrane protein with homology to transient receptor potential (TRP) channels, mediates the hyperosmolarity induced Ca2+ release. After this release, low cytosolic Ca2+ is restored and vacuolar Ca2+ is replenished through the activity of Vcx1p, a Ca2+/H+ exchanger. These studies reveal a novel mechanism of internal Ca2+ release and establish a new function for TRP channels.

SummaryThe effect of extracellular adenine and the role of the transcriptional activator Bas1p on expression of the yeast genome was assessed by two-dimensional (2D) analysis of the yeast proteome. These data combined with LacZ fusions and northern blot analysis allow us to show that synthesis of enzymes for all 10 steps involved in purine de novo synthesis is repressed in the presence of adenine and requires BAS1 and BAS2 for optimal expression. We also show that expression of ADE12 and ADE13, the two genes required for synthesis of AMP from inosine 5Јmonophosphate (IMP), is co-regulated with the de novo pathway genes. The same combined approach, used to study histidine biosynthesis gene expression, showed that HIS1 and HIS4 expression is co-regulated with purine biosynthesis genes whereas HIS2, HIS3, HIS5 and HIS6 expression is not. This work, together with previously published data, gives the first comprehensive overview of the regulation of purine and histidine pathways in a eukaryotic organism. Finally, the expression of two pyrimidine biosynthesis genes URA1 and URA3 was found to be severely affected by bas1 and bas2 mutations in the absence of adenine, establishing a regulatory link between the two nucleotide biosynthesis pathways.

The catalytic activity and intracellular localization of protein kinase C (PKC) are both highly regulated in vivo. This family of kinases contains conserved regulatory motifs, i.e., the C1, C2, and HR1 domains, which target PKC isoforms to specific subcellular compartments and restrict their activity spatially. Saccharomyces cerevisiae contains a single PKC isozyme, Pkc1p, which contains all of the regulatory motifs found in mammalian PKCs. Pkc1p localizes to sites of polarized growth, consistent with its main function in maintaining cell integrity. We dissected the molecular basis of Pkc1p localization by expressing each of its domains individually and in combinations as green fluorescent protein fusions. We find that the Rho1p-binding domains, HR1 and C1, are responsible for targeting Pkc1p to the bud tip and cell periphery, respectively. We demonstrate that Pkc1p activity is required for its normal localization to the bud neck, which also depends on the integrity of the septin ring. In addition, we show for the first time that yeast protein kinase C can accumulate in the nucleus, and we identify a nuclear exit signal as well as nuclear localization signals within the Pkc1p sequence. Thus, we propose that Pkc1p shuttles in and out of the nucleus and consequently has access to nuclear substrates. Surprisingly, we find that deletion of the HR1 domain results in Pkc1p localization to the mitotic spindle and that the C2 domain is responsible for this targeting. This novel nuclear and spindle localization of Pkc1p may provide a molecular explanation for previous observations that suggest a role for Pkc1p in regulating microtubule function.

Glutamine, glycine and 10-formyl tetrahydrofolate are consumed during de novo purine biosynthesis. We have found that, in Saccharomyces cerevisiae, synthesis of these cosubstrates is coregulated with synthesis of enzymes of the purine biosynthetic pathway. Analysis of three genes required for synthesis of glutamine, glycine and 10-formyl tetrahydrofolate (GLN1, SHM2 and MTD1, respectively) shows that their expression is repressed by adenine and requires the transcription factors Baslp and Bas2p. Northern analysis reveals that regulation of SHM2 and MTD1 expression by adenine takes place at the transcriptional level. We also show that Bas1p and Bas2p bind in vitro to the promoters of the SHM2 and MTD1 genes, and that mutations in the consensus Bas1p binding sequences strongly affect expression of these genes in vivo. Finally, we have found that a SHM2-lacZ fusion is expressed at a significantly higher level in a bas2-2 disrupted strain than in bas1-2 or bas1-2 bas2-2 mutant strains. The BAS1-dependent, BAS2-independent expression of SHM2-lacZ suggests that, in the absence of Bas2p, Bas1p can interact with another protein partner to activate SHM2 expression.