A single mRNA encodes the alpha 150 and alpha 80/70 forms of the alpha subunit of VLA4

VLA4 is a cell surface heterodimer (alpha 4 beta 1, CD49d/CD29) which belongs to the integrin family and is involved in cell-extracellular matrix interactions, as well as in intercellular adhesion. Unlike other integrin alpha subunits, the alpha subunit of VLA4 (alpha 4) can appear as a 150-kDa polypeptide (alpha 150), or cleaved into two non-covalently associated polypeptides (alpha 70-80). The relative proportion of each form is highly variable among different cells and is dependent on the state of cellular activation. The alpha 4 cleavage site has recently been shown to occur between residues Arg558-Ser559. We report the isolation of genomic clones encoding the alpha 4 subunit, the location of the cleavage site-encoding nucleotides to a specific exon and the determination of the exon-intron organization around the cleavage site. Comparison with the gpIIb (CD41) and the alpha x (p150,95 alpha, CD11c) genes revealed a similar genomic structure in this region, with exons of similar length separated by introns of identical phase. The structure of the alpha 4 mRNA in cells expressing the alternative forms of alpha 4 has been analyzed by means of reverse transcription-polymerase chain reaction. Our results indicate that the exon encoding the cleavage site is present in alpha 4 mRNA molecules from cells expressing either the alpha 150 or the alpha 70-80 form on the cell surface. Moreover, the structure of the alpha 4 mRNA around the cleavage site does not change during the switch towards the alpha 70-80 form that takes place upon lymphocyte activation. Therefore, both forms of alpha 4 arise from a common mRNA, the alpha 150 form contains the cleavage sequence and the alpha 70-80 form must be generated by a post-translational proteolytic event.