Anchorage of Candida albicans Ssr1 to the cell wall, and transcript profiling of the null mutant

Incorporation into the wall of Candida albicans Ssr1, a GPI-dependent protein, was investigated by construction of different truncated genes for which the three potential ω sites (S199, S215 and G216) and the corresponding ω+1 and ω+2 were eliminated or modified. Cells of the C. albicans ssr1Δ mutant were transformed with pADH-pl harboring the truncated versions of CaSSR1, pADH-ΔCaSSR1t(217-234) (lacking a C-terminal hydrophobic stretch of 18 aa including the putative ω+2 and ω+1, ω+2 of S215 and G216) or pADH-ΔCaSSR1t(199-201) (lacking three serine residues), and their walls were analyzed for the protein. Results suggested that the three serine residues are essential for incorporation of CaSsr1 into the wall β-glucan. This interpretation was confirmed when the truncated protein CaSsr1pt(199-201) was found in the spent medium. The transcription profile of the 6039 genes in C. albicans ssr1Δ showed that seven genes are upregulated (⩾1.4-fold), including SRP54 (a signal recognition particle subunit), IPF29 (a zinc finger protein) and PTR3 (a transcriptional regulator), whereas 27 genes are downregulated (⩽0.7-fold), including IPF6318 (a β-glucosidase) and SOU1 (a sorbitol utilization protein). Additional genes showed a reduced increase, or decreased expression, suggesting that some current orphan genes may have unknown cell wall functions. In addition, a compensatory mechanism would appear to occur, as a substantial increase in the amount of β-1,3-glucan (2.34-fold) was detected in the cell wall of the mutant cells.