Chemical cross-linking leads to two high molecular mass aggregates of rat alpha 1 beta 1 integrin differing in their conformation but not in their composition.

In order to detect protein interactions of the collagen/laminin receptor alpha 1 beta 1 integrin, covalent chemical cross-linking was performed with the homo-bifunctional, amine reactive reagents DSS (disuccinimidylsuberate) and DSP (dithiobis(succinimidylpropionate)). After cross-linking of the 190 kDa rat alpha 1 integrin subunit, immunoblotting revealed two additional, immunoreactive, high molecular mass complexes (M(r) 240/290 k). Generation of the 240/290 kDa aggregates depended on the presence of the intact tertiary protein structure. As shown with immunoaffinity purified proteins, the 240/290 kDa aggregates consist exclusively of alpha 1 and beta 1 integrin subunits. No other cross-linked proteins associated with the alpha 1 or beta 1 subunit were detected. In contrast to the non-cross-linkable alpha 1 beta 1 integrin, the 240/290 kDa aggregates presumably represent active forms of the adhesion receptor, because both bound in vitro to collagen I and IV. This ability of alpha 1 beta 1 integrin to cross-link and produce two additional high molecular mass forms is shared by rat alpha 9 beta 1 integrin. Thus, the cross-linking approach directly indicates that beta 1 integrins occur in different conformations caused by variations in the folding and/or spatial arrangement of their subunits.