Visualization of the large heparan sulfate proteoglycan from basement membrane

Abstract

Kleinschmidt spreading, negative staining, and rotary shadowing were used to examine the large form of (basement membrane) heparan suifate proteoglycan in the electron microscope. Heparan suifate proteoglycan was visualized as consisting of two parts: the core protein and, emerging from one end of the core protein, the glycosaminoglydan side chains. The core protein usually appeared as an S‐shaped rod with about six globules along its length. Similar characteristics were observed in preparations of core protein in which the side chains had been removed by heparitinase treatment (“400‐kDa core”) as well as in a 200‐kDa trypsin fragment (“P200”) derived from one end of the core protein. The core protein was sensitive to lyophilization and apparently also to the method of examination, being condensed following Kleinschmidt spreading (length x = 52 nm) and extended following negative staining (length x = 83 nm) or rotary shadowing (length x = 87 nm; 400‐kDa core length x = 80 nm; P200 length x = 44 nm). Two or three glyrosami‐noglycan side chains (length x = 146 + 53 nm) were attached to one end of the core protein. The side chains often appeared tangled or to merge together as one. Thus, the large heparan suifate proteoglycan from basement membrane is an asymmetrical molecule with a core protein containing globular domains and terminally attached side chains. This structure is in keeping with that previously predicted by enzymatic digestions and with the proposed orientation in basement membranes, i.e., the core protein bound in the lamina densa and the heparan suifate side chains in the lamina lucida arranged along the surface of the basement membranes.