The Yeast LATS/Ndr Kinase Cbk1 Regulates Growth via Golgi-dependent Glycosylation and Secretion

Abstract

Saccharomyces cerevisiae Cbk1 is a LATS/Ndr protein kinase and a downstream component of the regulation of Ace2 and morphogenesis (RAM) signaling network. Cbk1 and the RAM network are required for cellular morphogenesis, cell separation, and maintenance of cell integrity. Here, we examine the phenotypes of conditional cbk1 mutants to determine the essential function of Cbk1. Cbk1 inhibition severely disrupts growth and protein secretion, and triggers the Swe1-dependent morphogenesis checkpoint. Cbk1 inhibition also delays the polarity establishment of the exocytosis regulators Rab-GTPase Sec4 and its exchange factor Sec2, but it does not interfere with actin polarity establishment. Cbk1 binds to and phosphorylates Sec2, suggesting that it regulates Sec4-dependent exocytosis. Intriguingly, Cbk1 inhibition causes a >30% decrease in post-Golgi vesicle accumulation in late secretion mutants, indicating that Cbk1 also functions upstream of Sec2-Sec4, perhaps at the level of the Golgi. In agreement, conditional cbk1 mutants mislocalize the cis-Golgi mannosyltransferase Och1, are hypersensitive to the aminoglycoside hygromycin B, and exhibit diminished invertase and Sim1 glycosylation. Significantly, the conditional lethality and hygromycin B sensitivity of cbk1 mutants are suppressed by moderate overexpression of several Golgi mannosyltransferases. These data suggest that an important function for Cbk1 and the RAM signaling network is to regulate growth and secretion via Golgi and Sec2/Sec4-dependent processes. INTRODUCTIONCell polarity is a common cellular feature among eukaryotic cells that is characterized by asymmetry in cell shape, protein distribution, and cellular function (Nelson, 2003;Pruyne et al., 2004). The establishment and maintenance of polarized growth are critical for the development and function of specialized cells, tissues, and organs. Polarized growth is particularly important for the development and function of neuronal cells, which can reach several meters in length, and for epithelial cells, which regulate homeostasis and maintain physical barriers between biological compartments (Arimura and Kaibuchi, 2005;Dow and Humbert, 2007;Tang, 2008). Dynamic changes in cell polarity are essential for cell migration during cell and tissue development. Polarized growth and morphogenesis are also important for yeast and filamentous fungi development (Harris, 2006).Saccharomyces cerevisiae is an excellent model organism for studying conserved mechanisms of polarized growth and cell division. Saccharomyces cerevisiae cells undergo polarized growth during cell division, mating, and pseudohyphae formation (Pruyne et al., 2004). Many of the evolutionarily conserved elements of cell growth and proliferation were first identified and characterized in yeast (Pruyne et al., 2004). Yeast polarity establishment involves the recruitment and activation of Cdc42 GTPase to a cortical landmark. Cdc42 regulates several polarity processes, including septin assembly and actin cytoskeleton polarization (Park and Bi, 2007). Po...

The distinct wiring between cell cycle regulation and the widely conserved Morphogenesis-Related (MOR) pathway in the fungus Ustilago maydis determines the morphological outcome

“…Wee1 is a target of Ukc1) or indirect effect. It is interesting to note that previous research done in S. cerevisiae indicated that loss of Cbk1 function prevented bud emergence and caused a G2 arrest, which was the consequence of the activation of the morphogenesis checkpoint (Kurischko et al, 2008). In budding yeast, morphogenesis checkpoint operates by phosphorylation of Tyr15 of Cdc28 by Swe1, the S. cerevisiae homolog of Wee1 (Lew, 2003).…”

“…In budding yeast, morphogenesis checkpoint operates by phosphorylation of Tyr15 of Cdc28 by Swe1, the S. cerevisiae homolog of Wee1 (Lew, 2003). In fact, the G2 arrest consequence of Cbk1 inhibition in S. cerevisiae was prevented by deletion of Swe1 (Kurischko et al, 2008). One interesting possibility is that defects in MOR pathway in U. maydis triggers some sort of morphogenesis checkpoint and as a consequence the cell cycle is affected as it happens in S. cerevisiae.…”

Mutations in the Saccharomyces cerevisiae Kinase Cbk1p Lead to a Fertility Defect That Can Be Suppressed by the Absence of Brr1p or Mpt5p (Puf5p), Proteins Involved in RNA Metabolism

“…Recently Kurischko et al (2008) have shown that Cbk1p and the -2, 3-4, 5-7, 8-12, 13-15, 16-20, 21-29, and .29 cells. The mean and standard deviation between the three experiments are shown.…”

Lats kinase is involved in the intestinal apical membrane integrity in the nematodeCaenorhabditis elegans

“…Likewise, wts-1 might phosphorylate target proteins that regulate cellular integrity. Interestingly, Cbk1 in budding yeasts binds and phosphorylates Sec2, a guanyl-nucleotide exchange factor (GEF), which is involved in polarized growth and secretion in yeast (Kurischko et al, 2008). There are many GEF homologs in C. elegans, and it would be of interest to examine whether any of these can be phosphorylated by WTS-1.…”