Sbe2p and Sbe22p, two homologous Golgi proteins involved in yeast cell wall formation

The cell wall of fungal cells is important for cell integrity and cell morp hogenesis and protects against harmful environmental conditions. The yeast cell wall is a complex structure consisting mainly of mannoproteins, glucan , and chitin. The molecular mechanisms by which the cell wall components ar e synthesized and transported to the cell surface are poorly understood. We have identified and characterized two homologous yeast proteins, Sbe2p and Sbe22p, through their suppression of a chs5 spa2 mutant strain defective i n chitin synthesis and cell morphogenesis. Although sbe2 and sbe22 null mut ants are viable, sbe2 sbe22 cells display several phenotypes indicative of defects in cell integrity and cell wall structure. First, sbe2 sbe22 cells display a sorbitol-remediable lysis defect at 37 degrees C and are hypersen sitive to SDS and calcofluor. Second, electron microscopic analysis reveals that sbe2 sbe22 cells have an aberrant cell wall structure with a reduced mannoprotein layer. Finally, immunofluorescence experiments reveal that in small-budded cells, sbe2 sbe22 mutants mislocalize Chs3p, a protein involve d in chitin synthesis. Ln addition, sbe2 sbe22 diploids have a bud-site sel ection defect, displaying a random budding pattern. A Sbe2p-GFP fusion prot ein localizes to cytoplasmic patches, and Sbe2p cofractionates with Golgi p roteins. Deletion of CHS5, which encodes a Golgi protein involved in the tr ansport of Chs3p to the cell periphery, is lethal in combination with disru ption of SBE2 and SBE22. Thus, we suggest a model in which Sbe2p and Sbe22p are involved in the transport of cell wall components from the Golgi appar atus to the cell surface periphery in a pathway independent of Chs5p.