Alveolar basement membrane : molecular properties of the noncollagenous domain (hexamer) of collagen IV and its reactivity with goodpasture autoantibodies

The noncollagenous domain hexamer of collagen IV from bovine alveolar basement membrane was excised with bacterial collagenase, purified under nondenaturing conditions, and characterized. The hexamer is comprised of four distinct subunits [α1(IV)NCl, α2(IV)NCl, α3(IV)NCl, and α4(IV)NCl]. Each subunit exists in both monomeric and dimeric (disulfide-crosslinked) form, and both monomers and dimers have charge isoforms. Certain dimers also contain nonreducible crosslinks. The α3(IV)NCl subunit, in both the monomeric and dimeric form, reacts with Goodpasture (GP) antibodies. The GP epitope is sequestered within the hexamer and becomes reactive with antibody upon exposure with protein denaturants. These results reveal that the alveolar basement membrane hexamer is identical to the hexamer from glomerular basement membrane with respect to subunit composition, identity of subunits reacting with GP antibodies, and sequestration of the GP epitope but differs greatly in the relative amount of the GP-reactive subunit...