We report here the molecular cloning of cDNAs encoding for the human integrin alpha 1 subunit. The sequence is characteristic of an I domain containing integrin alpha subunit, with a high degree of homology to the rat integrin alpha 1 subunit, including complete identity of the transmembrane and cytoplasmic domains between the two species. The human cDNA directs the expression in mouse NIH 3T3 cells of authentic human alpha 1 protein as demonstrated by the reactivity of this subunit with two human-specific anti-alpha 1 monoclonal antibodies. This exogenous integrin specifically binds to type IV collagen in a Mg(2+)-dependent fashion. We have expressed in both transient systems and in stable cell lines truncated, soluble forms of the human alpha 1 subunit combined with truncated, soluble forms of beta 1 subunits. Although soluble beta 1 subunit was found in the media when the corresponding cDNA was used, the secretion of the soluble alpha 1 subunit was found to be dependent on dimerization with soluble beta 1. Co-transfection of truncated human alpha 1 cDNA with truncated forms of either the human or avian beta 1 cDNA led to efficient secretion of alpha 1.beta 1 heterodimers. These soluble heterodimers specifically bind to collagen IV in a manner similar to their full-length counterparts. Biosynthetic studies using stably expressing cell lines demonstrate that the soluble heterodimers and the native heterodimers are formed independently, strongly suggesting that the transmembrane or cytoplasmic domains of alpha and beta subunits are involved in the assembly of native heterodimers.