Cloning and sequence analysis of a novel β2related integrin transcript from T lymphocytes: homology of integrin cysteine-rich repeats to domain III of laminin B chains

Abstract

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.