G domains of the mouse laminin alpha 1 and alpha 4 chains consisting of its five subdomains LG1-LG5 were overexpressed in Chinese hamster ovary cells and purified by heparin chromatography. alpha 1LG1-LG5 and alpha 4LG1-LG5 eluted at NaCl concentrations of 0.30 and 0.47 m, respectively. In solid phase binding assays with immobilized heparin, half-maximal concentrations of 14 (alpha 1LG1-LG5) and 1.4 nm (alpha 4LG1-LG5) were observed. N-Glycan cleavage of alpha 4LG1-LG5 did not affect affinity to heparin. The affinity of alpha 4LG1-LG5 was significantly reduced upon denaturation with 8 m urea but could be recovered by removing urea. Chymotrypsin digestion of alpha 4LG1-LG5 yielded high and low heparin affinity fragments containing either the alpha 4LG4-LG5 or alpha 4LG2-LG3 modules, respectively. Trypsin digestion of heparin-bound alpha 4LG1-LG5 yielded a high affinity fragment of about 190 residues corresponding to the alpha 4LG4 module indicating that the high affinity binding site is contained within alpha 4LG4. Competition for heparin binding of synthetic peptides covering the alpha 4LG4 region with complete alpha 4LG1-LG5 suggests that the sequence AHGRL1521 is crucial for high affinity binding. Introduction of mutation of H1518A or R1520A in glutathione S-transferase fusion protein of the alpha 4LG4 module produced in Escherichia coli markedly reduced heparin binding activity of the wild type. When compared with the known structure of alpha 2LG5, this sequence corresponds to the turn connecting strands E and F of the 14-stranded beta-sheet sandwich, which is opposite to the proposed binding sites for calcium ion, alpha-dystroglycan, and heparan sulfate.