LAMININ INTERACTIONS IMPORTANT FOR BASEMENT-MEMBRANE ASSEMBLY ARE PROMOTED BY ZINC AND IMPLICATE LAMININ ZINC FINGER-LIKE SEQUENCES

Laminin is an abundant basement membrane (BM) glycoprotein which regul ates specific cellular functions and participates in the assembly and maintenance of the BM superstructure. The assembly of BM is believed t o involve the independent polymerization of collagen type IV and lamin in, as well as high affinity interactions between laminin, entactin/ni dogen, perlecan, and collagen type IV. We report here that Zn2+ can in fluence laminin binding activity, in vitro. Laminin contains 42 cystei ne-rich repeats of which 12 contained nested zinc finger consensus seq uences. Recently, the entactin binding site was mapped to one of these zinc finger-containing repeats on the laminin gamma chain (Mayer, U., Nischt, R., Poschl, E., Mann, K., Fukuda, K., Gerl, M., Yamada, Y., a nd Timpl, R. (1993) EMBO J. 12, 1879-1885), Based on these observation s, the effect of a series of essential ions (Ca2+, Cd2+, CU2+, Mg2+, M n2+, and Zn2+) on laminin binding activity was evaluated, Zn2+ was fou nd to be the most effective at enhancing laminin-entactin and laminin collagen type IV binding. Laminin-bound Zn2+ was detected by flame ato mic absorption spectroscopy at a maximum of 8 mol/mol of laminin, Furt hermore, Ca2+ dependent laminin polymerization was unaffected by Zn2+. an observation consistent with the lack of zinc finger-containing rep eats in the terminal globular domains required fbr polymerization, We conclude that Zn2+-laminin complexes may generate high affinity bindin g sites which contribute to BM cross-linking important for its assembl y and homeostasis. Zinc is likely a cofactor for 2 kinds of cross-link ing interactions; one involving direct binding between laminin and col lagen type IV and the other a ternary complex of lamininentactin-colla gen type IV.