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

CAP superfamily proteins, also known as sperm-coating 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.