Epithelial Integrin O / 6 ~ 4 : Complete Primary Structure of and Variant Forms of ~ 4

The integrin ~6/34 is a heterodimer predominantly expressed by epithelia. While no definite receptor function has yet been assigned to it, this integrin may mediate adhesive and/or migratory functions of epithelial cells. We have determined the complete primary structure of both the as and/34 subunits from cDNA clones isolated from pancreatic carcinoma cell line libraries. The deduced amino acid sequence of or6 is homologous to other integrin o~ chains (18-26% identity). Antibodies to an a6 carboxy terminus peptide immunoprecipitated 0t6~4 complexes from carcinoma cells and 0t6/31 complexes from platelets, providing further evidence for the association of ~6 with more than one t3 subunit. The deduced amino acid sequence of/34 predicts an extracellular portion homologous to other integrin/3 chains, and a unique cytoplasmic domain comprised of > 1,000 residues. This agrees with the structures of the ~4 cDNAs from normal epithelial cells (Suzuki, S., and Y. Naitoh. 1990. EMBO [Eur. Mol. Biol. Organ.] J. 9:757-763; Hogervost, E, I. Kuikman, A. E. G. Kr. von dem Borne, and A. Sonnenberg. 1990. EMBO [Eur. Mol. Biol. Organ.] J. 9:765-770). Compared to these structures, however, the /~4 cDNAs that we have cloned from carcinoma cells contain extra sequences. One of these is located in the 5'-untranslated region, and may encode regulatory sequences. Another specifies a segment of 70 amino acids in the cytoplasmic tail. Amplification by reverse transcription-polymerase chain reaction of mRNA indicated that multiple forms of ~4 may exist, possibly due to cell-type specific alternative splicing. The unique structure of/34 suggests its involvement in novel cytoskeletal interactions. Consistent with this possibility, 0t6~4 is mostly concentrated on the basal surface of epithelial cells, but does not colocalize with components of adhesion plaques. T HE interaction of cells with the extracellular matrix is important for the formation, maintenance, and repair of tissues as well as for other biological processes such as the metastasis of cancer cells. This interaction is mediated, in part, by a family of cell surface receptors called integrins (Hynes, 1987; Ruoslahti and Pierschbacher, 1987; Buck and Horwitz, 1987). These receptors form a link between the extracellular matrix and the cytoskeleton and may transmit signals from the extracellular to the intracellular environment that affect cell behavior. Integrins are heterodimers comprised of a and/3 subunits, that are noncovalently associated transmembrane glycoproteins. At least 11 0/chains (Ruostahti and Giancotti, 1989) and 6/3 chains (Sheppard et al., 1990) have been recognized in man. Each 0/subunit tends to associate with only one type of/3 subunit, but there are several exceptions to this rule (Hemler et al., 1989; Cheresh et al., 1989; Holzmann et al., 1989; Freed et al., 1989). This is publication No. 6533-IMM from the Research Institute of Scripps Clinic. We and others have previously reported the identification of an integrin expressed primarily on epithelial-type cells, termed 0/E/34 or 0d6/34 (Kajiji et al., 1987, 1989; Hemler et al., 1989). The alpha subunit, 0/6, also appears to associate with the/3, integrin subunit and, on platelets, u6/3~ has been shown to function as a laminin receptor (Sonnenberg et al., 1988). Here we report the complete cDNA sequence and the deduced amino acid sequence of both the 0/6 and the /34 subunits. Both 0/6 and /34 are homologous with the other integrin 0/and/3 chains, respectively. However, they contain unique structural features which may suggest novel functional properties. Materials and Methods