MAPPING OF NETWORK-FORMING, HEPARIN-BINDING, AND ALPHA-1-BETA-1 INTEGRIN-RECOGNITION SITES WITHIN THE ALPHA-CHAIN SHORT ARM OF LAMININ-1

Cell-interactive and architecture-forming functions are associated wit h the short arms of basement membrane laminin-1. To map and characteri ze these functions, we expressed recombinant mouse laminin-1 alpha-cha in extending from the N terminus through one third of domain IIIb. Thi s dumbbell-shaped glycoprotein (r alpha 1(VI-IVb)'), secreted by mamma lian cells, was found to possess three activities. 1) Laminin polymeri zation was quantitatively inhibited by recombinant protein, supporting an alpha-chain role for a three-short arm interaction model of lamini n self-assembly. 2) r alpha 1(VI IVb)' bound to heparin, and the activ ity was localized to a subfragment corresponding to domain VI by I-125 -heparin blotting. 3) PC12 rat pheochromocytoma cells adhered to, and rapidly extended branching neurites on, r alpha 1(VI-IVb)', with adhes ion inhibited by alpha 1 and beta 1 integrin chain-specific antibodies . The ability of anti-laminin antibody to block PC12 cell adhesion to laminin was selectively prevented by absorption with r alpha 1(VI-IVb) ' or alpha-chain domain VI fragment. This active integrin-recognition site could furthermore be distinguished from a second cryptic alpha 1 beta 1-binding site exposed by heat treatment of fragment P1', a short arm fragment lacking globules. Thus, a polymer-forming, a heparin-bin ding, and the active alpha 1 beta 1 integrin-recognition site are all clustered at the end of the alpha-chain short arm, the latter two resi dent solely in domain VI.