Comprehensive Analysis of Glycosylphosphatidylinositol-Anchored Proteins in Candida albicans

There are two types of membrane proteins: the integral membrane proteins and the lipid-anchored proteins. Integral membrane proteins contain one or several transmembrane do- mains that allow for the formation of hydrophobic -helices, which ultimately embed the protein in a lipid bilayer. We count four types of lipid-anchored proteins divided into two groups: one group generally links the proteins to the internal side of the plasma membrane and the other generally links them to the outside. In the first group are found the amide-linked myristoyl anchors (calcineurin B) (6), the (thio)ester-linked fatty acyl anchors, and the thioether-linked prenyl anchors (G protein alpha subunits) (13). The second group is composed only of the proteins that contain a C-terminal signal sequence that allows for linkage to a glycosylphosphatidylinositol (GPI) anchor. The anchor of the GPI-anchored proteins (GpiPs) can be removed by the action of specific phospholipases (24), con- verting the protein into a water-soluble form. GpiPs are thought to associate with lipid rafts, specialized regions of the membrane containing elevated levels of cholesterol and sphin- golipids, common to human cells, for example (48). Proteins destined to be GPI anchored share conserved features: an N-terminal signal sequence for localization to the endoplasmic reticulum (ER), a C-terminal hydrophobic domain (9 to 24 residues) for transient attachment to the ER membrane, and the so-called site, where the protein is cleaved to be ligated to the GPI anchor (57). The site is localized 9 to 10 amino acids before the C-terminal hydrophobic domain, and its amino acid environment determines whether the protein has a high probability to be GPI anchored. Amino acids at positions -1 to -11 form a linker region usually with no charge and no secondary structure (-helix or -sheet). The site region is composed of small amino acids to fit the transamidase protease catalytic pocket, and finally, the spacer region ( 3t o9) is comparable to the linker, having no charge and being flexible (16). Shortly after protein synthesis in the ER, the preformed GPI anchor replaces the C-terminal transmembrane region. The core GPI anchor consists of a lipid group (serving as the membrane anchor), a myoinositol group, an N-acetylglu- cosamine group, three mannose groups, and a phosphoethanol-