Cell surface proteoglycan that bears heparan sulfate. Binds, via the heparan sulfate side chains, alpha-4 (V) collagen and participates in Schwann cell myelination (By similarity). May act as a catalyst in increasing the rate of conversion of prion protein PRPN (C) to PRNP (Sc) via associating (via the heparan sulfate side chains) with both forms of PRPN, targeting them to lipid rafts and facilitating their interaction. Required for proper skeletal muscle differentiation by sequestering FGF2 in lipid rafts preventing its binding to receptors (FGFRs) and inhibiting the FGF-mediated signaling (By similarity). S-nitrosylated form recycled in endosomes. Localizes to CAV1-containing vesicles close to the cell surface. Cleavage of heparan sulfate side chains takes place mainly in late endosomes. Associates with both forms of PRNP in lipid rafts. Colocalizes with APP in perinuclear compartments and with CP in intracellular compartments (By similarity). S-nitrosylated in a Cu(2+)-dependent manner. Nitric acid (NO) is released from the nitrosylated cysteines by ascorbate or by some other reducing agent, in a Cu(2+) or Zn(2+) dependent manner. This free nitric oxide is then capable of cleaving the heparan sulfate side chains (By similarity). N- and O-glycosylated (By similarity). N-glycosylation is mainly of the complex type containing sialic acid. O-glycosylated with heparan sulfate. The heparan sulfate chains can be cleaved either by the action of heparanase or, degraded by a deaminative process that uses nitric oxide (NO) released from the S-nitrosylated cysteines. This process is triggered by ascorbate, or by some other reducing agent, in a Cu(2+)- or Zn(2+) dependent manner. Cu(2+) ions are provided by ceruloproteins such as APP, PRNP or CP which associate with GCP1 in intracellular compartments or lipid rafts (By similarity). Shed from the cell surface probably by further cleavage. Belongs to the glypican family.