Molecular analysis reveals localization of Saccharomyces cerevisiae protein kinase C to sites of polarized growth and Pkc1p targeting to the nucleus and mitotic spindle

KlRHO1 and KlPKC1 are essential for cell integrity signalling in Kluyveromyces lactis

Cell integrity in yeasts is ensured by a rigid cell wall whose synthesis is triggered by a MAP kinase-mediated signal-transduction cascade. Upstream regulatory components of this pathway inSaccharomyces cerevisiaeinvolve a single protein kinase C, which is regulated by interaction with the small GTPase Rho1. Here, two genes were isolated which encode these proteins fromKluyveromyces lactis(KlPKC1andKlRHO1). Sequencing showed ORFs which encode proteins of 1161 and 208 amino acids, respectively. The deduced proteins shared 59 and 85 % overall amino acid identities, respectively, with their homologues fromS. cerevisiae. Null mutants in both genes were non-viable, as shown by tetrad analyses of the heterozygous diploid strains. Overexpression of theKlRHO1gene under the control of theScGAL1promoter severely impaired growth in bothS. cerevisiaeandK. lactis. On the other hand, a similar construct withKlPKC1did not show a pronounced phenotype. Two-hybrid analyses showed interaction between Rho1 and Pkc1 for theK. lactisproteins and theirS. cerevisiaehomologues. A green fluorescent protein (GFP) fusion to the C-terminal end of KlPkc1 located the protein to patches in the growing bud, and at certain stages of the division process also to the bud neck. N-terminal GFP fusions to KlRho1 localized mainly to the cell surface (presumably the cytoplasmic side of the plasma membrane) and to the vacuole, with some indications of traffic from the former to the latter. Thus, KlPkc1 and KlRho1 have been shown to serve vital functions inK. lactis, to interact in cell integrity signalling and to traffic between the plasma membrane and the vacuole.

Cell wall assembly in Saccharomyces cerevisiae

An extracellular matrix composed of a layered meshwork of beta-glucans, chitin, and mannoproteins encapsulates cells of the yeast Saccharomyces cerevisiae. This organelle determines cellular morphology and plays a critical role in maintaining cell integrity during cell growth and division, under stress conditions, upon cell fusion in mating, and in the durable ascospore cell wall. Here we assess recent progress in understanding the molecular biology and biochemistry of cell wall synthesis and its remodeling in S. cerevisiae. We then review the regulatory dynamics of cell wall assembly, an area where functional genomics offers new insights into the integration of cell wall growth and morphogenesis with a polarized secretory system that is under cell cycle and cell type program controls.

A spatiotemporally coordinated cascade of protein kinase C activation controls isoform-selective translocation

In pituitary GH3B6 cells, signaling involving the protein kinase C (PKC) multigene family can self-organize into a spatiotemporally coordinated cascade of isoform activation. Indeed, thyrotropin-releasing hormone (TRH) receptor activation sequentially activated green fluorescent protein (GFP)-tagged or endogenous PKCbeta1, PKCalpha, PKCepsilon, and PKCdelta, resulting in their accumulation at the entire plasma membrane (PKCbeta and -delta) or selectively at the cell-cell contacts (PKCalpha and -epsilon). The duration of activation ranged from 20 s for PKCalpha to 20 min for PKCepsilon. PKCalpha and -epsilon selective localization was lost in the presence of Gö6976, suggesting that accumulation at cell-cell contacts is dependent on the activity of a conventional PKC. Constitutively active, dominant-negative PKCs and small interfering RNAs showed that PKCalpha localization is controlled by PKCbeta1 activity and is calcium independent, while PKCepsilon localization is dependent on PKCalpha activity. PKCdelta was independent of the cascade linking PKCbeta1, -alpha, and -epsilon. Furthermore, PKCalpha, but not PKCepsilon, is involved in the TRH-induced beta-catenin relocation at cell-cell contacts, suggesting that PKCepsilon is not the unique functional effector of the cascade. Thus, TRH receptor activation results in PKCbeta1 activation, which in turn initiates a calcium-independent but PKCbeta1 activity-dependent sequential translocation of PKCalpha and -epsilon. These results challenge the current understanding of PKC signaling and raise the question of a functional dependence between isoforms.

The biochemical effect of Ser167 phosphorylation on Chlamydomonas reinhardtii centrin

Centrin is an EF-hand calcium-binding protein found in microtubule organizing centers of organisms ranging from algae and yeast to man. Phosphorylation in the centrin C-terminal domain occurs in mitosis and is associated with alterations in contractile fibers. To obtain insight into the structural basis for the functional effect of phosphorylation, Chlamydomonas reinhardtii centrin C-terminal domain phosphorylated at Ser167 (pCRC-C) has been produced and characterized. The structure of pCRC-C was compared to the unmodified protein by NMR spectroscopy. The effect of phosphorylation on target binding was examined for the complex of pCRC-C and a 19 residue centrin-binding fragment of Kar1. Remarkably, the efficient and selective phosphorylation by PKA was suppressed in the complex. Moreover, comparisons of NMR chemical shift differences induced by phosphorylation reveal a greater effect from phosphorylation in the context of the Kar1 complex than for the free protein. These results directly demonstrate that phosphorylation modulates the structure and biochemical activities of centrin.

The identification of a functional interaction between PKC and topoisomerase II

Topoisomerase II plays an essential role in the segregation of chromosomes during cell division. It is also a major component of the nuclear matrix. Proteins that interact with and regulate this essential enzyme are of great interest. To investigate the role of proteins interacting with the N-terminal domain of the Saccharomyces cerevisiae topoisomerase II, we used a yeast two-hybrid protein interaction screen. We identified an interaction between the catalytic domain of the yeast protein kinase 1 enzyme (Pkc1) and the N-terminal domain of the S. cerevisiae topoisomerase II. The S. cerevisiae Pkc1 is the homologue of the mammalian calcium dependent PKC.

Baker's yeast as a tool for the development of antifungal kinase inhibitors—targeting protein kinase C and the cell integrity pathway

Today, the yeast Saccharomyces cerevisiae is probably the best-studied eukaryotic organism. This review first focuses on the signaling process which is mediated by the unique yeast protein kinase C (Pkc1p) and a downstream mitogen-activated protein kinase (MAPK) cascade. This pathway ensures cellular integrity by sensing cell surface stress and controlling cell wall biosynthesis and progression through the cell cycle. The domain structure of Pkc1p is conserved from yeast to humans. A yeast system for heterologous expression of specific domains in a chimeric yeast/mammalian PKC enzyme ("domain shuffling") is depicted. It is also proposed how this system could be employed for the study of protein kinase inhibitors in high-throughput screens. Moreover, a reporter assay that allows a quantitative readout of the activity of the cell integrity signaling pathway is introduced. Since a variety of protein kinases take part in the signal transduction, this broadens the range of targets for potential inhibitors.

Cell wall integrity signaling in Saccharomyces cerevisiae

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca(2+) signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.