Heparan sulfate proteoglycan is present in basement membrane as a double-tracked structure.

Basement membranes contain 4.5-nm wide sets of two parallel lines, along which short prongs called "spikes" occur at regular intervals. The nature of this structure, referred to as "double tracks," was investigated in Lowicryl sections of mouse kidney and rat Reichert's membrane immunolabeled for basement membrane components using secondary antibodies conjugated to 5-nm gold particles. When the mouse glomerular basement membrane and rat Reichert's membrane were exposed to antibodies directed to the core protein of heparan sulfate proteoglycan, 95% or more of the gold particles were over double tracks, whereas after exposure of Reichert's membrane to antisera against laminin, collagen IV, or entactin, labeling of the double tracks remained at the random level. When heparan sulfate proteoglycan was incubated in Tris buffer, pH 7.4, at 35 degrees C for 1 hr, a precipitate resulted which, on electron microscopic examination, was found to consist of 5- to 6-nm wide sets of two parallel lines along which densities were observed. Immunolabeling confirmed the presence of the proteoglycan's core protein in the sets. Since double tracks were closely similar to this structure and were labeled with the same antibodies, they were likely to be also composed of heparan sulfate proteoglycan.