cDNA cloning reveals two mouse β5 integrin transcripts distinct in cytoplasmic domains as a result of alternative splicing

The integrin β5 subunit has only been found to form a heterodimer with subunit αv which acts as a vitronectin receptor. Integrin αvβ5 has been implicated in cell migration and growth factor-induced angiogenesis. In the present study, a mouse liver cDNA library was screened using a human β5 cDNA fragment obtained by reverse transcriptase PCR (RT-PCR). Three of the clones (MB5, MB15 and MB17) overlapped to give an open reading frame, called β5A, which is homologous to the human β5 subunit. The sequence of another clone (MB26), called β5B, was identical with β5A, except for a deletion of 29 bp near the 3´ end of the open reading frame. The 29 bp deletion resulted in an open-reading-frame shift and a completely different C-terminal sequence in β5B. β5A and β5B were shown, by RT-PCR, to be co-expressed in most mouse tissues tested, although β5B mRNA was detected at much lower levels than β5A. β5A and β5B mRNAs were also detected in the mouse monocytic cell line, J774, and in isolated mouse peritoneal macrophages. Adhesion of peritoneal macrophages has been shown to up-regulate the expression of both β5A and β5B mRNAs. The 29 bp sequence begins with a putative intron-splicing donor site (GTGAT …). A 3´ fragment of the mouse integrin β5 gene was cloned by PCR and sequenced showing that the 29 bp sequence was also immediately followed by an intron. Therefore, the 29 bp sequence was apparently expressed as part of the β5A mRNA but was spliced out as part of the downstream intron in β5B. Since the cytoplasmic domains of the integrin β subunits are important in cytoskeleton attachment and signalling, the two alternatively spliced β5 isoforms may have distinct roles in cell adhesion and other cellular functions.