The CAP protein superfamily: function in sterol export and fungal virulence

Abstract

CAP superfamily proteins, also known as spermcoating proteins, are found in all kingdoms of life and have been implicated in a variety of physiological contexts, including immune defense in plants and mammals, sperm maturation and fertilization, fungal virulence, and toxicity of insect and reptile venoms as well as prostate and brain cancer. CAP family members are mostly secreted glycoproteins that are highly stable in the extracellular fluid. All members of the superfamily share a common CAP domain of approximately 150 amino acids, which adopts a unique α-β-α sandwich fold. The conserved structure suggests that CAP proteins exert fundamentally similar functions. However, the molecular mode of action of this protein family has remained enigmatic. The budding yeast Saccharomyces cerevisiae has three CAP family members designated Pry (pathogen related in yeast), and recent evidence indicates that they act as sterol-binding and export proteins. Expression of the mammalian CAP protein CRISP2, which binds sterols in vitro, complements the sterol export defect of a yeast pry mutant, suggesting that sterol binding and export is conserved among different CAP family members. Collectively, these observations suggest that CAP family members constitute a novel class of secreted extracellular sterol-binding proteins. A ligand-binding activity of the CAP domain could explain many of the biological activities attributed to these proteins. For example, the strong induction of plant pathogenesis-related 1 protein upon exposure to pathogens may serve to inhibit pathogen proliferation by extracting sterols from the pathogen membrane. Similarly, the presence of these proteins in the venom of toxic insects and reptiles or in the secretome of pathogenic fungi might inflict damage by sequestering sterols or related small hydrophobic compounds from the host tissue.

The caveolin-binding motif of the pathogen-related yeast protein Pry1, a member of the CAP protein superfamily, is required for in vivo export of cholesteryl acetate

“…In agreement with this proposition, the CAP domain of Pry1 itself is necessary and suffi cient for sterol export in vivo and for cholesterol binding in vitro ( 10 ). Moreover, expression of the human superfamily member, CRISP2, rescues the sterol export defect of yeast cells lacking Pry function and purifi ed CRISP2 binds cholesterol in vitro, suggesting that CAP superfamily members may generally act as lipid-binding proteins ( 10,11 ).…”

“…This protein-lipid interaction is specifi c, as it is lost when a highly conserved cysteine residue that is known to form a disulfi de bridge is mutated to serine. The lipid-binding and export function of the Pry proteins appears to be a conserved function of the members of the CAP protein superfamily, because expression of the human CAP protein, CRISP2, relieves the lipid export block of a yeast mutant lacking Pry function, and the purifi ed CRISP2 binds sterols in vitro ( 10,11 ).…”

The pathogen-related yeast protein Pry1, a member of the CAP protein superfamily, is a fatty acid-binding protein

“…6). These results thus indicate that Pry1 can bind polyunsaturated fatty acids, such as arachidonic acids (C20-5, 8,11,14), which has its second double bond at position 8 of the acyl chain. Mono unsaturated fatty acids, on the other hand, contain their double bond at position 9 of the acyl chain and do not bind to Pry1.…”

“…The sterol-binding and export function of yeast Pry proteins appears to be a conserved function of members of the CAP protein superfamily since expression of the human CAP protein CRISP2 complements the defect in sterol export of a yeast mutant lacking Pry function, and purified CRISP2 binds sterols in vitro (13,14). The capacity to bind sterols is also conserved in SmVal4, a CAP protein from the human parasite Schistosoma mansoni, as well as in PR-1, the founding member of the CAP superfamily from plants (15,16).…”

Plant pathogenesis–related proteins of the cacao fungal pathogen Moniliophthora perniciosa differ in their lipid-binding specificities

“…Sterol binding by Pry proteins requires the displacement of a flexible loop containing aromatic amino acids, termed the caveolin-binding motif (CBM) within the CAP domain, since point mutations within this motif abolish sterol export and binding, whereas fatty acid binding takes place in the groove between two parallel running helices, 1 and 3 (19,24). Taken together, these data indicate that CAP proteins may exert an immunomodulatory function through binding hydrophobic ligands, such as sterols or related small hydrophobic compounds and fatty acids and their derivatives such as leukotrienes (19,23,25).…”