The globular domain IVa (about 250 residues) of the laminin α1 chain was obtained in recombinant form from mammalian cell clones. It was prepared either with (α1IVa-R) or without (α1IVa) an adjacent cell-adhesive RGD site which seems to be masked in laminin-1. The recombinant products could be visualized as globular structures by rotary shadowing, were resistant to trypsin and shared immunological epitopes with laminin-1, indicating folding into a native structure. Sequence analysis of pepsin fragments demonstrated the insertion of the globular domain into an epidermal growth factor-like scaffold which is characteristic of the extracellular laminin domain IV (L4) module. Only little immunological cross-reaction was found, however, with other L4 modules from perlecan and different laminin isoforms. Fragment α1IVa-R, but not fragment α1IVa, bound to αVβ3 integrin, although to a distinctly lower level than a laminin fragment where the RGD site is fully exposed. The fragments also had no or only little cell attachment activity. This confirmed previous predictions that the globular domain α1IVa masks the RGD site in laminin-1. Domain α1IVa showed, in addition, a weak binding activity for the basement-membrane protein fibulin-1.

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