Biochemical characterization of the major peanut-agglutinin-binding glycoproteins in vertebrate retinae

Peanut agglutinin (PNA), a lectin that binds D-galactose-beta (1----3) N-acetyl-D-galactosamine disaccharide linkages, selectively labels cone photoreceptors in the retinae of a variety of species. PNA binds consistently to domains of the interphotoreceptor matrix associated with cone, but not rod, inner and outer segments, to cone cell body and axonal membranes, to cone synaptic pedicles, and to portions of the inner plexiform layer. In order to begin the characterization of the molecular species responsible for cone-specific PNA binding, chick, turkey, rat, dog, pig, monkey, and human retinal extracts were separated by SDS-polyacrylamide gel electrophoresis and probed with peroxidase-conjugated PNA. The results reveal the presence of six major groups of PNA-binding glycoproteins ranging from 30 to 88 kilodaltons. Most of these are shared by the seven species examined; however, some interspecies variation is present. Three groups, designated GP39/40, GP42/45, and GP60, are the most intensely labeled by PNA and are common to all species analyzed, while groups GP29/31 and GP88 are less intensely labeled and are present in most but not all of the species investigated. Labeling of the GP54 group is variable but is most consistently associated with extracts of rat and pig retinae. Trypsin treatment, which results in the loss of cone-associated PNA binding in the interphotoreceptor matrix, causes a visually detectable reduction in three of the six groups of PNA-binding glycoproteins in porcine retinal extracts. Of these, GP54 is the most sensitive, being undetectable on PNA-stained blots after only 5 minutes of enzyme exposure; GP88 and GP45 are less sensitive but both are markedly reduced after 15 minutes of trypsinization. Trypsin-sensitive molecules thus may be involved in the establishment of the cone-specific domains of interphotoreceptor matrix identified by PNA binding. These, as well as the other groups of PNA-binding molecules, are being utilized to develop more specific immunologic probes with which to further study of their distribution and function.