No abstract

No abstract

The enzyme that catalyzes the synthesis of the major structural component of the yeast cell wall, beta(1-->3)-D-glucan synthase (also known as 1,3-beta-glucan synthase), requires a guanosine triphosphate (GTP) binding protein for activity. The GTP binding protein was identified as Rho1p. The rho1 mutants were defective in GTP stimulation of glucan synthase, and the defect was corrected by addition of purified or recombinant Rho1p. A protein missing in purified preparations from a rho1 strain was identified as Rho1p. Rho1p also regulates protein kinase C, which controls a mitogen-activated protein kinase cascade. Experiments with a dominant positive PKC1 gene showed that the two effects of Rho1p are independent of each other. The colocalization of Rho1p with actin patches at the site of bud emergence and the role of Rho1p in cell wall synthesis emphasize the importance of Rho1p in polarized growth and morphogenesis.

No abstract

Chitin is a minor but essential component of the cell wall of Saccharomyces cerevisiae, with functions in septum formation in the vegetative life cycle and also in conjugation and spore cell-wall synthesis in the sexual cycle. Of the three chitin synthases present in yeast, chitin synthase III (CSIII) is responsible for the synthesis of most of the chitin found in the cell, including a chitin ring at early budding, chitin interspersed in the cell wall, and chitin laid down during the sexual cycle. We have tagged Chs3p, the putative catalytic subunit of CSIII, with the immunoreactive epitope of influenza virus hemagglutinin to follow expression of the protein. Little correlation was found between the levels of transcription and translation of Chs3p and in vivo function, supporting our previous conclusion that regulation of CSIII occurs at the posttranslational level. To identify possible regions of the protein involved in catalysis or regulation, mutations were generated in the QRRRW 'signature sequence' of chitin synthases. Arginine residue mutations in Chs3p, and in Chs1p and Chs2p, resulted in a loss of both function in vivo and enzymatic activity. Mutations in a serine residue adjacent to glutamine in Chs3p caused loss of function in vivo with a moderate decrease in CSIII activity, suggesting a regulatory role for the serine residue in chitin biosynthesis. Several truncations in the unique hydrophilic carboxy-terminal region of Chs3p identified a sequence of about 25 amino acids that is required for both function and in vitro activity. Since this region is not present in Chs1 or Chs2, it may be involved in the specific regulation of CSIII.Keywords : chitin synthase III; calcofluor resistance; morphogenesis; Saccharomyces cerevisiae.Chitin is a component of the yeast cell wall that, although only present in small amounts, is required for cell viability and has a major role in septum formation and cell division [1Ϫ3]. Three chitin synthases have been described in Saccharomyces cerevisiae, CSI, CSII and CSIII (see [1Ϫ3] for reviews), each with different functions. CSIII is responsible for synthesis of the chitin ring laid down at the base of an emerging bud; CSII is subsequently involved in the formation of the primary septum disk ; finally, CSI repairs damaged chitin during cell separation. Since CSIII also catalyzes the synthesis of chitin interspersed over all the cell wall and of that laid down during mating and sporulation, this enzyme is responsible for production of most of the chitin found in the cell [1].The different roles of chitin synthases require that each be regulated independently, so that it will execute its function at the required time and location in the cell. This regulation may take place during expression of the protein and/or posttranslationally. Some evidence of transcriptional regulation of the genes coding for the putative catalytic subunits of the three synthases (CHS1, CHS2 and CHS3) has been found [4]. However, further work showed that only CSII activity oscillates in the cell cycle...