Self-assembly of a high molecular weight basement membrane heparan sulfate proteoglycan into dimers and oligomers.

A high molecular weight basement membrane heparan sulfate proteoglycan, isolated from murine Englebreth-Holm-Swarm tumor, is seen in platinum replicas as an elongated flexible core (Mr = 450,000) consisting of a series of tandem globular domains from which extend, at one end, two to three heparan sulfate chains (average Mr = 80,000 each). This macromolecule will self-assemble into dimers and lesser amounts of oligomers when incubated in neutral isotonic buffer. These molecular species can be separated by zonal velocity sedimentation and assembly is seen to be time- and concentration-dependent. In rotary-shadowed platinum replicas the binding region is found at or near the end of the core at the pole opposite the origin of the heparan sulfate chains. Dimers are double-length structures and oligomers are seen as stellate clusters: in both, the heparan sulfate chains appear peripherally oriented. While isolated cores self-assemble, isolated heparan sulfate chains do not bind intact proteoglycans. Furthermore, proteolytic removal of a non-heparan sulfate containing core moiety destroys the ability of the proteoglycan monomer to form larger species or bind intact proteoglycan, further supporting the binding topography determined morphologically. These negatively charged macromolecular complexes may be important contributors to basement membrane structure and function.