The non-collagenous domains of the alpha 3 and 4 chains of type IV collagen and their relationship to the goodpasture antigen

The Goodpasture antigen is the target recognized by anti-glomerular basement membrane (GBM) antibodies in anti-GBM disease or Goodpasture's syndrome. This structure is present in all normal GBM, but when serum containing anti-GBM antibodies is used to examine renal tissue from most males with classical Alport's syndrome, the Goodpasture antigen appears to be missing. The nature of the Goodpasture antigen is uncertain although it has been putatively and controversially localized to the non-collagenous domain of a novel type IV collagen chain (alpha 3) by one group, and a short peptide sequence has been published (M2). We have performed several experiments to determine whether M2 represents the Goodpasture antigen and we have also studied the corresponding sequence of the alpha 4 chain of type IV collagen (M3). Firstly, we demonstrated by polymerase chain reaction (PCR) amplification using specific priming oligonucleotides that mRNAs corresponding to M2 and M3 were found within the kidney and that the published sequences were correct. When heterologous antibodies were raised against M2 and M3 these bound specifically to GBM in an ELISA based on collagenase-digested basement membrane and this binding could be inhibited by incubation with collagenase-digested GBM but not with ovalbumin. On further examination of the target molecules using Western blots, the anti-M2 antibody bound to a single high molecular weight band of collagen-digested GBM in contrast to the anti-M3 antibody that bound to the same bands as Goodpasture serum. We then established ELISAs for anti-M2 and anti-M3 activity using the peptides M2 and M3. While rabbit anti-M2 and M3 antibodies bound specifically to their respective peptides in these ELISAs, there was no binding of three high titre Goodpasture's syndrome sera or two sera from Alport's syndrome patients with inhibitable anti-GBM antibody post-renal transplant. We have shown that the sequences of M2 and M3 correspond to proteins present within the collagenase-resistant part of the GBM, suggesting that these do represent parts of novel type IV collagen chains. However, sera containing anti-GBM antibodies did not bind to either peptide in solid-phase ELISAs, and these antibodies may recognize a different peptide sequence, features of the tertiary structure of these peptides or interactions between collagen chains.