Cloning and sequence analysis of a novel beta 2-related integrin transcript from T lymphocytes: homology of integrin cysteine-rich repeats to domain III of laminin B chains.

The integrins have been grouped into three subfamilies based on the utilization of three distinct beta chain subunits, beta 1, beta 2, and beta 3. The recent discovery of beta 4 and beta 5 subunits, and the sharing of alpha subunits between beta subfamilies reflects a greater structural and functional diversity of the integrin supergene family than first anticipated. We exploit integrin gene sequence homology for the identification of a new integrin beta chain. Oligonucleotide probes designed from the highly conserved Arg-Gly-Asp (RGD)-binding domain were used to amplify related transcripts from phytohaemagglutinin-activated human peripheral blood lymphocytes using the polymerase chain reaction (PCR). Five PCR products encoding beta 1, beta 2, beta 3, beta 4, and a new beta clone, designated beta 7, were identified. Full-length beta 7 cDNA was isolated from an activated human T lymphocyte library, using an oligonucleotide probe constructed from the beta 7 PCR product. The beta 7 cDNA contains a long open reading frame of 2391 bp which encodes a protein sequence consisting of 797 amino acids. The encoded sequence revealed a typical signal peptide, a predominantly hydrophilic 707 amino acid residue domain with 8 N-glycosylation sites, a transmembrane domain, and a C-terminal domain of 52 amino acids. The beta 7 sequence showed homology to known beta 1, beta 2, beta 3, beta 4, and beta 5 sequences of 43, 46, 38, 32, and 37% respectively. Four cysteine-rich homologous repeat sequences were found in beta 7 and were homologous to sequences in other integrin beta subunits, and to domain III of the laminin B chains. Part of this cysteine-rich region is homologous to proteins that contain epidermal growth factor-like repeat sequences. Northern analysis revealed that mature beta 7 mRNA is approximately 3.5 kb in size, and expression was restricted to T and B lymphocytes in the small panel of cell types examined. We conclude that beta 7 may be a new member of the leukocyte cell adhesion molecule subset of integrins, and is a candidate immunoregulatory molecule.