Characterization of protease-resistant fragments of laminin mediating attachment and spreading of rat hepatocytes

In attempts to identify cell binding domains in the basement membrane protein laminin (M(r) = 900,000), responsible for the substrate attachment mediating activity of the protein, proteolytic fragments were isolated and compared with respect to their biological activity. The fragments analyzed were generated by digestion of the protein with elastase or pepsin and included the previously described fragments 1 to 4 and three additional fragments, 5, 6 and 7 with molecular weights of 30,000 to 50,000. Fragments 1, 5, and 6 promoted substrate attachment of rat hepatocytes. Fragment 5, and to a lesser extent also fragment 6, but not fragment 1, induced spreading of the cells. Other fragments including the heparin-binding domain were inactive. Structural and immunological analyses indicated that fragment 1 is distinctly different from the other cell-binding fragments, whereas fragment 5 and 6 are similar but not identical. Furthermore, the active fragments were localized to different regions of the three short arms of the cross-shaped laminin molecule. Thus, the data suggest that fragments 1, 5, and 6 represent three separate domains with cell binding capacity. Attachment of hepatocytes to the fragments but not to intact laminin could be inhibited by specific antibodies indicating that the intact protein may contain an additional cell-binding site.