Human and baboon integrin beta 5 subunit-encoding mRNAs have alternative polyadenylation sites

cDNAs encoding the baboon thrombin receptor indicate a primate transcription start site upstream of putative sites reported for the human gene

Two cDNAs encoding the thrombin receptor of baboon vascular smooth muscle cells have 5'-untranslated regions that begin upstream of multiple putative transcription initiation sites reported for the closely related human receptor gene. The extent of these baboon 5'-untranslated cDNA regions and their close similarity to the corresponding human sequences suggest that there is only one transcription initiation site of the primate thrombin receptor gene, which might be linked to a typical TATA-box previously identified in the upstream region of the human gene. It is possible that all primates have a unique thrombin receptor gene transcription start site. Inferences drawn from the baboon system may be usefully extrapolated to the human, in view of the extensive similarities seen between the nucleotide sequences of baboon and human thrombin receptor gene sequences in the 5'- untranslated and coding regions. The extents of the 5'-untranslated region of the baboon cDNAs argue that the "multiple" transcription start sites identified for the human gene are artifactual. The striking differences in the reported baboon and human transcription start sites warrant further investigation in view of the significant role played by the thrombin receptor in numerous vascular and cellular growth responses.

cDNA cloning reveals two mouse beta5 integrin transcripts distinct in cytoplasmic domains as a result of alternative splicing

The integrin beta5 subunit has only been found to form a heterodimer with subunit alphav which acts as a vitronectin receptor. Integrin alphavbeta5 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 beta5 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 beta5A, which is homologous to the human beta5 subunit. The sequence of another clone (MB26), called beta5B, was identical with beta5A, 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 beta5B. beta5A and beta5B were shown, by RT-PCR, to be co-expressed in most mouse tissues tested, although beta5B mRNA was detected at much lower levels than beta5A. beta5A and beta5B 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 beta5A and beta5B mRNAs. The 29 bp sequence begins with a putative intron-splicing donor site (GTGAT...). A 3' fragment of the mouse integrin beta5 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 beta5A mRNA but was spliced out as part of the downstream intron in beta5B. Since the cytoplasmic domains of the integrin beta subunits are important in cytoskeleton attachment and signalling, the two alternatively spliced beta5 isoforms may have distinct roles in cell adhesion and other cellular functions.