Candida albicans cell wall proteins

Abstract

The Candida albicans cell wall maintains the structural integrity of the organism in addition to providing a physical contact interface with the environment. The major components of the cell wall are fibrillar polysaccharides and proteins. The proteins of the cell wall are the focus of this review. Three classes of proteins are present in the candidal cell wall. One group of proteins attach to the cell wall via a glycophosphatidylinositol remnant or by an alkali-labile linkage. A second group of proteins with N-terminal signal sequences but no covalent attachment sequences are secreted by the classical secretory pathway. These proteins may end up in the cell wall or in the extracellular space. The third group of proteins lack a secretory signal, and the pathway(s) by which they become associated with the surface is unknown. Potential constituents of the first two classes have been predicted from analysis of genome sequences. Experimental analyses have identified members of all three classes. Some members of each class selected for consideration of confirmed or proposed function, phenotypic analysis of a mutant, and regulation by growth conditions and transcription factors are discussed in more detail.

FIG. 1.

Electron microscopy of C. albicans cell wall. (A) Immunoelectron microscopy of thin sections reacted with antibody recognizing Pir1p and then with gold-conjugated second antibody showing protein presence within the cell wall. (Reprinted from reference with permission from Blackwell Publishing.) (B) Electron microscopy of cells reacted with antibody to Als1p followed by reaction with gold-conjugated second antibody before embedding and sectioning. (Courtesy of Lois Hoyer, reproduced with permission.) (C) Freeze-fractured, freeze-etched hydrophilic cell of C. albicans showing fibrils at the outer surface. F, fracture face. Bar, 0.19 μm. (Reprinted from reference with permission.) (D) Immunoelectron microscopy of thin sections reacted with antibody recognizing Hsp70 and then with gold-conjugated secondary antibody showing protein presence within the cell wall. Arrows indicate a series of gold particles. Bar, 0.2 μm. (Reprinted from reference .)

FIG. 2.

Schematic representation of major cell wall components. The cell wall is external to the cell membrane (shown in black and white at the bottom). Labeled symbols: red rectangles, GPI-CWPs; yellow hexagons, Pir proteins. Unlabeled symbols: dark blue lines, β-1,3-glucan; medium blue lines, β-1,6-glucan; light blue lines, chitin; maroon circles, phosphomannolipid; green circles, unattached proteins found in the cell wall or in the medium.