Evidence for the presence of a low-mass β1 integrin on the cell surface

Although the cell line K562 reportedly expresses a single species of β1 integrin, α5β1, surface staining with monoclonal antibodies JB1A, 12G10 and B3B11 to the β1 chain clearly demonstrated differences in the expression levels of the epitopes detected by these antibodies. The present studies were initiated to determine the basis for this molecular heterogeneity in the integrins. Cross-linking of surface integrins with B3B11 caused their selective aggregation. This distribution was similar to that observed for the α5 chain. In contrast, cross-linking the β1 chains with 12G10 did not cause codistribution of α5, suggesting that these two species were not associated on the cell surface. Immunoprecipitates of the surface integrins of K562 cells indicated the presence of 120 and 140 kDa forms of the β1 chain which were detected by 12G10 and B3B11, respectively. Immunological, biochemical and mass spectrometric analysis of K562 surface integrins also failed to demonstrate the presence of any α chain in association with the 120 kDa species of β1 of K562 cells. Treatment of the two forms of β1 with PGNase reduced their masses to ∼90 kDa, suggesting that N-glycosylation was responsible for the mass differences. Collectively, these results provide evidence for a novel species of β1 on the cell surface, which does not appear to be associated with any α chain. The data also suggest that differences in glycosylation may be involved in defining the association between the integrin α and β chains and the functional properties of these integrins.