Cell surface and substrate distribution of the 67-kDa laminin-binding protein determined by using a ligand photoaffinity probe

Abstract

BACKGROUND: Peptide 11, a nine-amino acid sequence from the beta1 chain of laminin-1, has been reported to inhibit tumor cell invasion of basement membranes, and to reduce tumor lung colonization (Iwamoto et al.: Science 238:1132-1134, 1987; Landowski et al.: Clin Exp Metastasis 13:357-372, 1995). The peptide is a ligand for the 32/67-kDa laminin-binding protein (LBP); however, the mechanism by which the 67-kDa LBP promotes invasion is unknown. METHODS: We have synthesized a highly specific probe for the 67-kDa LBP by adding a biotinylated residue, and replacing the required tyrosine in peptide 11 with the photoactivatable bezophenone crosslinker, 4-benzoyl-L-phenylalanine. This probe was used to follow the distribution of the 67-kDa LBP by gel electrophoresis, fluorescence-activated cell scanning, and confocal microscopy techniques. RESULTS: A single crosslinked protein, consistent with the high molecular weight form of the LBP, was found on Western blots of membrane detergent extracts from cells treated with the ligand probe. A CHO cell line, manipulated to overexpress the laminin-specific alpha6beta1 integrin, exhibited increased invasiveness, and expressed more cell surface 67-kDa LBP. Membrane-associated 67-kDa LBP was found in the vicinity of focal adhesion plaques and also associated with the matrix substrate. Studies on conditioned medium indicated that the matrix-associated LBP derived from material that was shed from the cells, with more being shed from the more invasive CHO variants. CONCLUSIONS: These results demonstrate the utility of this novel probe in diverse experimental protocols, and suggest that shedding of the 67-kDa LBP may have a role in promoting tumor cell invasion.