Abstract. We analyzed the binding of fibronectin to integrin a5/31 in various cells; in some cells fibronectin bound with low affinity (e.g., K562 cells) whereas in others (e.g., CHO), it bound with high affinity (Kd ,~ 100 nM) in an energy-dependent manner. We constructed chimeras of the extracellular and transmembrane domains of c~3 joined to the cytoplasmic domains of otsBl. The affinity state of these chimeras was assessed by binding of fibrinogen or the monoclonal antibody, PACl. The cytoplasmic domains of 0~5/~ conferred an energy-dependent high affinity state on otnbB3 in CliO but not K562 cells. Three additional tx cytoplasmic domains (ix2, otv~, ot~B) conferred PAC1 binding in CHO cells, while three others (otM, OiL, av) did not. In the high affinity ot chimeras, cotransfection with a truncated (~3A724) or mutated (~3(S752"-~P)) 83 subunit abolished high affinity binding. Thus, both cytoplasmic domains are required for energy-dependent, cell type-specific affinity modulation. In addition, mutations that disrupted a highly conserved o~ subunit GFFICR motif, resulted in high affinity binding of ligands to Otnb/~3. In contrast to the chimeras, the high affinity state of these mutants was independent of cellular metabolism, cell type, and the bulk of the ~ subunit cytoplasmic domain. Thus, integrin cytoplasmic domains mediate inside-out signaling. Furthermore, ' the highly conserved GFFKR motif of the c~ Subunit cytoplasmic domain maintains the default low affinity state.

A new member of the integrin superfamily of adhesion receptors was isolated from human epithelial cells. Analogously to other integrins, this molecule is a heterodimer comprised of structurally unrelated subunits, both glycosylated. Unequivocal amino‐acid sequence homologies were observed between these subunits and integrin alpha and beta chain sequences, indicating that this epithelial heterodimer is a novel integrin. No obvious serologic cross‐reactivities were detected with other integrins. The beta chain of the epithelial integrin displayed a mol. wt significantly higher than other integrin beta chains, possibly due to a large sialic acid content. Integrin heterodimers are grouped into three families, based on which of three beta chains (beta 1, beta 2 and beta 3) they contain. Therefore, the epithelial integrin may represent the prototype of a fourth integrin family, because it contains a structurally distinct beta chain. The designation alpha E beta 4 is proposed for this novel human integrin.

Epithelial cell interactions with matrices are critical to tissue organization. Indirect immunofluorescence and immunoprecipitations of cell lysates prepared from stratified cultures of human epidermal cells showed that the major integrins expressed by keratinocytes are ad3,4 (also called a644) and a2pl/a3.l1. The adP4 integrin is localized at the surface of basal cells in contact with the basement membrane, whereas a2fil/a3fl1 integrins are absent from the basal surface and are localized only on the lateral surface of basal and spinous keratinocytes. Anti-fl4 antibodies potently inhibited keratinocyte adhesion to matrigel or purified laiinin, whereas anti-fl1 antibodies were ineffective. Only anti-fl4 antibodies were able to detach established keratinocyte colonies. These data suggest that aEP4 mediates keratinocyte adhesion to basal lamina, whereas the ,BI subfamily is involved in cell-cell adhesion of keratinocytes.

The integrin heterodimers composed of the a6 subunit with the j31 or (84 subunit (a68l1 and a6(34)