Laminin interactions important for basement membrane assembly are promoted by zinc and implicate laminin zinc finger-like sequences

Abstract

Laminin is an abundant basement membrane (BM) glycoprotein which regulates specific cellular functions and participates in the assembly and maintenance of the BM superstructure. The assembly of BM is believed to involve the independent polymerization of collagen type IV and laminin, as well as high affinity interactions between laminin, entactin/nidogen, perlecan, and collagen type IV. We report here that Zn2+ can influence laminin binding activity, in vitro. Laminin contains 42 cysteine-rich repeats of which 12 contained nested zinc finger consensus sequences. Recently, the entactin binding site was mapped to one of these zinc finger-containing repeats on the laminin γ chain (Mayer, U., Nischt, R. Poschl, E., Mann, K., Fukuda, K. Gerl, M. Yamada, Y., and Timpl, R. (1993) EMBO J. 12, 1879-1885). Based on these observations, the effect of a series of essential ions (Ca2+, Cd2+, Cu2+, Mg2+, Mn2+, and Zn2+) on laminin binding activity was evaluated. Zn2+ was found to be the most effective at enhancing laminin- entactin and laminin-collagen type IV binding. Laminin-bound Zn2+ was detected by flame atomic absorption spectroscopy at a maximum of 8 mol/mol of laminin. Furthermore, Ca2+-dependent laminin polymerization was unaffected by Zn2+, an observation consistent with the lack of zinc finger-containing repeats in the terminal globular domains required for polymerization. We conclude that Zn2+-laminin complexes may generate high affinity binding sites which contribute to BM cross-linking important for its assembly and homeostasis. Zinc is likely a cofactor for 2 kinds of cross-linking interactions; one involving direct binding between laminin and collagen type IV and the other a ternary complex of laminin-entactin-collagen type IV.