The high-affinity binding of laminin to cells. Assignation of a major cell-binding site to the long arm of laminin and of a latent cell-binding site to its short arms.

The laminin proteolytic fragments 1 (derived from the intersection of the short arms of the cruciform laminin molecule) and 8 (derived from the laminin long arm) bind to distinct receptors on HT-1080 human fibrosarcoma cells; both fragments are shown here to inhibit the high-affinity binding of laminin to these cells. Inhibition of binding between fragment 8 and laminin was competitive, whereas that between fragment 1 and laminin was noncompetitive. This indicates that laminin and fragment 8 most probably share the same cellular receptors, whereas laminin and fragment 1 bind to distinct receptors, inhibition being due to steric hindrance. Surprisingly, fragment 1-4 (corresponding to the complete short arms of laminin) neither bound to HT-1080 cells nor inhibited the binding of laminin or fragment 1. After treatment of fragment 1-4 with pepsin, however, the smaller subfragment 1 was liberated, which could then bind to the cells, and so was shown to block the binding of laminin and fragment 1. We conclude that native laminin bound to HT-1080 cells via the fragment-8-binding site near the end of its long arm. Although these cells also have distinct receptors for the short arm fragment 1, this receptor-binding site was not used as it appeared to be latent within the native laminin molecule.