SPACR, A NOVEL INTERPHOTORECEPTOR MATRIX GLYCOPROTEIN IN HUMAN RETINATHAT INTERACTS WITH HYALURONAN

SPACR (sialoprotein associated with cones and rods), is the major 147- 150-kDa glycoprotein present in the insoluble interphotoreceptor matri x of the human retina. Immunocytochemistry localizes SPACR to the matr ix surrounding rods and cones (Acharya, S., Rayborn, M. E., and Hollyf 'ield, J. G. (1998) Glycobiology 8, 997-1006), From affinity-purified SPACR, we obtained seven peptide sequences showing 100% identity to th e deduced sequence of IMPG1, a purported chondroitin 6-sulfate proteog lycan core protein, which binds peanut agglutinin and is localized to the interphotoreceptor matrix. We show here that SPACR is the most pro minent 147-150-kDa band present in the interphotoreceptor matrix and i s the gene product of IMPG1. SPACR is not a chondroitin sulfate proteo glycan, since it is not a product of chondroitinase ABC digestion and does not react to a specific antibody for chondroitin B-sulfate proteo glycan. Moreover, the deduced amino acid sequence reveals no establish ed glycosaminoglycan attachment site. One hyaluronan binding motif is present in the predicted sequence of SPACR. We present evidence that S PACR has a functional hyaluronan binding domain, suggesting that inter actions between SPACR and hyaluronan may serve to form the basic macro molecular scaffold, which comprises the insoluble interphotoreceptor m atrix.