FIGURE 2. Depiction of HSPGs associated with cell surfaces.

The syndecan and glypican family members are integral membrane proteins. The syndecan core proteins are apparently highly extended transmembrane proteins that contain a short (34–38-residue) carboxy-terminal cytoplasmic domain. The HS chains on the syndecans are linked to serine residues that are distal from the plasma membrane. The glypican core proteins are apparently disulphide-stabilized globular core proteins linked to the plasma membrane by an apparently extended C-terminal region that culminates in a GPI linkage. The HS chains on the glypicans are linked to serine residues adjacent to the plasma membrane in the apparently extended region. Basement membranes in mammalian tissues contain perlecan and agrin. Perlecan is in nearly all basement membranes as well as in cartilage, whereas agrin is a component of basement membranes in muscle, myoneural junctions, the central nervous system, lung and kidney. Both are large multidomain proteins bearing HS chains near their amino termini. Although both proteins exist as multiple isoforms that arise by alternative splicing and contain globular domains that are homologous to laminin G and epidermal growth factor domains, on the basis of their genomic organization, they are quite distinct proteins.