SPACRCAN, a novel human interphotoreceptor matrix hyaluronan-binding proteoglycan synthesized by photoreceptors and pinealocytes

The interphotoreceptor matrix is a unique extracellular complex occupying t he interface between photoreceptors and the retinal pigment epithelium in t he fundus of the eye. Because of the putative supportive role in photorecep tor maintenance, it is likely that constituent molecules play key roles in photoreceptor function and may be targets for inherited retinal disease. In this study we identify and characterize SPACRCAN, a novel chondroitin prot eoglycan in this matrix. SPACRCAN was cloned from a human retinal cDNA libr ary and the gene localized to chromosome 3q11.2. Analysis of SPACRCAN mRNA and protein revealed that SPACRCAN is expressed exclusively by photorecepto rs and pinealocytes, SPACRCAN synthesized by photoreceptors is localized to the interphotoreceptor matrix where it surrounds both rods and cones, The functional protein contains 1160 amino acids with a large central mucin dom ain, three consensus sites for glycosaminoglycan attachment, two epidermal growth factor-like repeats, a putative hyaluronan-binding motif, and a pote ntial transmembrane domain near the C-terminal, Lectin and Western blotting indicate an M-r around 400,000 before and 230,000 after chondroitinase ABC digestion. Removal of N- and O-linked oligosaccharides reduces the M-r to approximately 160,000, suggesting that approximately 60% of the mass of SPA CRCAN is carbohydrate. Finally, we demonstrate that SPACRCAN binds hyaluron an and propose that associations between SPACRCAN and hyaluronan may be inv olved in organization of the insoluble interphotoreceptor matrix, particula rly as SPACRCAN is the major proteoglycan present in this matrix.