Protein kinase C (PKC) signaling is highly conserved among eukaryotes and has been implicated in the regulation of cellular processes such as cell proliferation and growth. In the budding yeast, PKC1 functions to activate the SLT2(MPK1) mitogen-activated protein (MAP) kinase cascade, which is required for the maintenance of cell integrity during asymmetric cell growth. Genetic studies, coimmunoprecipitation experiments, and analysis of protein phosphorylation in vivo and in vitro indicate that the SBF transcription factor (composed of Swi4p and Swi6p), an important regulator of gene expression at the G1 to S phase cell cycle transition, is a target of the Slt2p(Mpk1p) MAP kinase. These studies provide evidence for a direct role of the PKC1 pathway in the regulation of the yeast cell cycle and cell growth and indicate that conserved signaling pathways can act to control key regulators of cell division.
Retinoic acid (RA) metabolites of vitamin A are key regulators of gene expression involved in embryonic development and maintenance of epithelial tissues. The cellular effects of RA are dependent upon the complement of nuclear receptors expressed (RARs and RXRs), which transduce retinoid signals into transcriptional regulation, the presence of cellular retinoid-binding proteins (CRABP and CRBP), which may be involved in RA metabolism, and the activity of RA metabolizing enzymes. We have been using the zebrafish as a model to study these processes. To identify genes regulated by RA during exogenous RA exposure, we utilized mRNA differential display. We describe the isolation and characterization of a cDNA, P450RAI, encoding a novel member of the cytochrome P450 family. mRNA transcripts for P450RAI are expressed normally during gastrulation, and in a defined pattern in epithelial cells of the regenerating caudal fin in response to exogenous RA. In COS-1 cells transfected with the P450RAI cDNA, alltrans-RA is rapidly metabolized to more polar metabolites. We have identified 4-oxo-RA and 4-OH-RA as major metabolic products of this enzyme. P450RAI represents the first enzymatic component of RA metabolism to be isolated and characterized at the molecular level and provides key insight into regulation of retinoid homeostasis.
NuA4, the only essential histone acetyltransferase complex in Saccharomyces cerevisiae , acetylates the N-terminal tails of histones H4 and H2A. Affinity purification of NuA4 revealed the presence of three previously undescribed subunits, Vid21/Eaf1/Ydr359c, Swc4/Eaf2/Ygr002c, and Eaf7/Ynl136w. Experimental analyses revealed at least two functionally distinct sets of polypeptides in NuA4: ( i ) Vid21 and Yng2, and ( ii ) Eaf5 and Eaf7. Vid21 and Yng2 are required for bulk histone H4 acetylation and are functionally linked to the histone H2A variant Htz1 and the Swr1 ATPase complex (SWR-C) that assembles Htz1 into chromatin, whereas Eaf5 and Eaf7 have a different, as yet undefined, role. Mutations in Htz1, the SWR-C, and NuA4 cause defects in chromosome segregation that are consistent with genetic interactions we have observed between the genes encoding these proteins and genes encoding kinetochore components. Because SWR-C-dependent recruitment of Htz1 occurs in both transcribed and centromeric regions, a NuA4/SWR-C/Htz1 pathway may regulate both transcription and centromere function in S. cerevisiae .
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