Peptidase family S41 (C-terminal processing peptidase or CTPase family) contains very different subfamilies; it includes photosystem II D1 C-terminal processing protease (CTPase), interphotoreceptor retinoid-binding protein IRBP and tricorn protease (TRI). CTPase and TRI both contain the PDZ domain while IRBP, although being very similar to the tail-specific protease domain, lacks the PDZ insertion domain and hydrolytic activity. These serine proteases have distinctly different active sites: in CTPase, the active site consists of a serine/lysine catalytic dyad while in tricorn core protease, it is a tetrad (serine, histidine, serine, glutamate). CPases with different substrate specificities in different species include processing of D1 protein of the photosystem II reaction center in higher plants and cleavage of a peptide of 11 residues from the precursor form of penicillin-binding protein; and others such as tricorn protease (TRI) act as a carboxypeptidase, involved in the degradation of proteasomal products. CTPase homolog IRBP, secreted by photoreceptors into the interphotoreceptor matrix, having arisen in the early evolution of the vertebrate eye, promotes the release of all-trans retinol from photoreceptors and facilitates its delivery to the retinal pigment epithelium.
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