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CAP (cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins) domain family The CAP (cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins) domain, also called SCP (sperm-coating glycoprotein), is found in eukaryotes and prokaryotes. This family includes plant pathogenesis-related protein 1 (PR-1), which accumulates after infections with pathogens, and may act as an anti-fungal agent or be involved in cell wall loosening. This family also includes CRISPs (cysteine-rich secretory proteins), which combine the CAP/SCP domain with a C-terminal cysteine rich domain, and allergen 5 from vespid venom. Roles for CRISP, in response to pathogens, fertilization, and sperm maturation have been proposed. One member, Tex31 from the venom duct of Conus textile, has been shown to possess proteolytic activity sensitive to serine protease inhibitors. The human GAPR-1 protein has been reported to dimerize, and such a dimer may form an active site containing a catalytic triad. CAP/SCP has also been proposed to be a Ca++ chelating serine protease. The Ca++-chelating function would fit with various signaling processes that members of this family, such as the CRISPs, are involved in, and is supported by sequence and structural evidence of a conserved pocket containing two histidines and a glutamate. It also may explain how helothermine, a toxic peptide secreted by the beaded lizard, blocks Ca++ transporting ryanodine receptors. Little is known about the biological roles of the bacterial and archaeal CAP/SCP domains. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",