A large chondroitin sulfate basement membrane-associated proteoglycan exists as a disulfide-stabilized complex of several proteins.
Proteoglycan (PG)-1000 (formerly TAP-1) is a large (Mr = 10(6)) highly glycosylated chondroitin sulfate proteoglycan found associated with Schwann cell and electrocyte basement membranes in elasmobranch electric fish. Previously, purified PG-1000 was visualized in the electron microscope as a "bottlebrush" structure about 345 nm long with about 20 side projections of 113 nm. This molecule was characterized with material purified from electric organ under denaturing and reducing conditions. Here we report that PG-1000, when purified under denaturing conditions without exposure to a reducing agent, exists as a complex of several proteins. In addition to PG-1000, this complex consists of a somewhat smaller, heavily glycosylated protein (beta component) and three smaller proteins with Mr values of 39,000, 21,000, and 18,000. The complex remains intact when exposed to denaturing and non-reducing conditions but falls apart in denaturing and reducing conditions. Presumably the complex is stabilized by disulfide bonds. The beta component of the PG-1000 complex is probably a proteoglycan. However, unlike PG-1000, the beta component does not contain chondroitin sulfate chains and lacks the epitope, T1, that is found on PG-1000. Both molecules share a protease-insensitive antigenic site, SV4, which is probably a modified keratan sulfate epitope. Evidence for the identity of this antigen is that it is found as a minor subfraction in commercial preparations of shark cartilage chondroitin and corneal keratan sulfates but not in other glycosaminoglycan preparations. These SV4 antigens are resistant to chondroitin ABC lyase digestion. However, the SV4 antigen in commercial keratan sulfate is cleaved by keratinase to a smaller antigenic fragment.