Migration of Macrophages

Several integrin a subunits have structural variants that are identical in their extracellular and transmembrane domains but that differ in their cytoplasmic domains. The functional significance of these variants, however, is unknown. In the present study, we examined the possibility that the A and B variants of the a6f1 integrin laminin receptor differ in function. For this purpose, we expressed the a6A and a6B cDNAs, as well as a truncated a6 cDNA (a6-ACYT) in which the cytoplasmic domain sequence was deleted after the GFFKR pentapeptide, in P388D1 cells, an a6 deficient macrophage cell line. Populations of stable a6A, a6B, and a6-ACYT transfectants that expressed equivalent levels of cell surface a6 were obtained by fluorescence-activated cell sorter and shown to form heterodimers with endogenous ,B1 subunits. Upon attachment to laminin, the a6A transfectants extended numerous pseudopodia. In contrast, the a6B transfectants remained rounded and extended few processes. The transfectants were also examined for their ability to migrate toward a laminin substratum using Transwell chambers. The a6A transfectants were threeto fourfold more migratory than the a6B transfectants. The a6-ACYT transfectants did not attach to laminin in normal culture medium, but they did attach in the presence of Mn2". The a6-ACYT transfectants migrated to a lesser extent than either the a6A or a6B transfectants in the presence of Mn2". The a6 transfectants differed significantly in the concentration of substratum bound laminin required for half-maximal adhesion in the presence of Mn2+:a6A (2.1 ,ug/ml), a6B (6.3 ,g/ml), and a6-ACYT (8.8 ,ug/ml). Divalent cation titration studies revealed that these transfectants also differed significantly in both the [Ca2"] and [Mn2"] required to obtain half-maximal adhesion to laminin. These data demonstrate that the A and B variants of the a6 cytoplasmic domain can differentially modulate the function of the a6f31 extracellular domain.