Protein kinase C enables the regulatory circuit that connects membrane synthesis to ribosome synthesis in Saccharomyces cerevisiae

The balanced growth of a cell requires the integration of major systems such as DNA replication, membrane biosynthesis, and ribosome formation. An example of such integration is evident from our recent finding that, in Saccharomyces cerevisiae, any failure in the secretory pathway leads to severe repression of transcription of both rRNA and ribosomal protein genes. We have attempted to determine the regulatory circuit(s) that connects the secretory pathway with the transcription of ribosomal genes. Experiments show that repression does not occur through the circuit that responds to misfolded proteins in the endoplasmic reticulum, nor does it occur through circuits known to regulate ribosome synthesis, e.g. the stringent response, or the cAMP pathway. Rather, it appears to depend on a stress response at the plasma membrane that is transduced through protein kinase C (PKC), Deletion of PKC1 relieves the repression of both ribosomal protein and rRNA genes that occurs in response to a defect in the secretory pathway. We propose that failure of the secretory pathway prevents the synthesis of new plasma membrane. As protein synthesis continues, stress develops in the plasma membrane. This stress is monitored by Pkc1p, which initiates a signal transduction pathway that leads to repression of transcription of the rRNA and ribosomal protein genes. The importance of the transcription of the 137 ribosomal protein genes to the economy of the cell is apparent from the existence of at least three distinct pathways that can effect the repression of this set of genes.