Saccharomyces cerevisiae YCRO17c/CWH43 encodes a putative sensor/transporter protein upstream of the BCK2 branch of the PKCI-dependent cell wall integrity pathway

The Saccharomyces cerevisiae cwh43-2 mutant, originally isolated for its Ca lcofluor white hypersensitivity, display's several cell wall defects simila r to mutants in the PKC1-MPK1 pathway, including a growth defect and increa sed release of beta -1,6-glucan and beta -glucosylated proteins into the gr owth medium at increased temperatures. The cloning of CWH43 showed that it corresponds to YCR017c and encodes a protein with 14-16 transmembrane segme nts containing several putative phosphorylation and glycosylation sites. Th e N-terminal part of the amino acid sequence of Cwh43p shows 40%, similarit y with the mammalian FRAG1, a membrane protein that activates the fibroblas t growth factor receptor of rat osteosarcoma (FCFR2-ROS) and with protein s equences of four uncharacterized ORFs from Caenorhabditis elegans and one f rom Drosophila melanogaster. The C-terminus of Cwh43p shows low similaritie s with a xylose permease of Bacillus megaterium and with putative sugar tra nsporter from D. melanogaster, and has 52% similarity with a protein sequen ce from a Schizosaccharomyces pombe cDNA. A Cwh43-GFP fusion protein sugges ted a plasma membrane localization, although localization to the internal s tructure of the cells could not be excluded, and it concentrates to the bud tip of small budded cells and to the neck of dividing cells. Deletion of C WH43 resulted in cell wall defects less pronounced than those of the cwh43- 2 mutant. This allele-specific phenotype appears to be due to a G-R substit ution at position 57 in a highly conserved region of the protein, Genetic a nalysis places CWH43 upstream of the BCK2 branch of the PKC1 signalling pat hway, since cwh43 mutations were synthetic lethal with pkcl deletion, where as the cwh43 defects could be rescued by overexpression of BCK2 and not by high-copy-number expression of genes encoding downstream proteins of the PK C1 pathway. However, unlike BCK2, whose disruption in a cln3 mutant resulte d in growth arrest in G(1), no growth defect was observed in a double cwh43 cln3 mutants, Taken together, it is proposed that CWH43 encodes a protein with putative sensor and transporter domains acting in parallel to the main PKC1-dependent cell wall integrity pathway, and that this gene has evolved into two distinct genes in higher eukaryotes. Copyright (C) 2001 John Wile y & Sons, Ltd.