The α Chain of Laminin-1 is Independently Secreted and Drives Secretion of Its β - and γ -Chain Partners

A mammalian recombinant strategy was established to dissect rules of basement membrane laminin assembly and secretion. The α -, β -, and γ -chain subunits of laminin-1 were expressed in all combinations, transiently and/or stably, in a near-null background. In the absence of its normal partners, the α chain was secreted as intact protein and protein that had been cleaved in the coiled-coil domain. In contrast, the β and γ chains, expressed separately or together, remained intracellular with formation of β β or β γ , but not γ γ , disulfide-linked dimers. Secretion of the β and γ chains required simultaneous expression of all three chains and their assembly into α β γ heterotrimers. Epitope-tagged recombinant α subunit and recombinant laminin were affinity-purified from the conditioned medium of α γ and α β γ clones. Rotary-shadow electron microscopy revealed that the free α subunit is a linear structure containing N-terminal and included globules with a foreshortened long arm, while the trimeric species has the typical four-arm morphology of native laminin. We conclude that the α chain can be delivered to the extracellular environment as a single subunit, whereas the β and γ chains cannot, and that the α chain drives the secretion of the trimeric molecule. Such an α -chain-dependent mechanism could allow for the regulation of laminin export into a nascent basement membrane, and might serve an important role in controlling basement membrane formation.