Poly-immunoglobulin receptor

Homology to immunoglobulin units is often weak whilst /3-strand patterns are suggestive of V units, only Thy-1. the membrane-distal unit of MRC OX-2 and the poly-immunoglobulin receptor units possess the extra hairpin loop (C /C") which is characteristic of V units (Fig. 11). Thus, the polypeptide chain may fold in a similar way to C units. Although all these molecules are believed to exist as monomers, in most cases the possibility of homodimeric associations has not been ex-... 

The poly-Ig receptor has been cloned and sequenced [89] to reveal that the poly-Ig binding portion, i.e. secretory component, is composed of five highly conserved domains of approximately 100 amino acids which show considerable homology with immunoglobulin domains. It is possible that this arrangement of secretory components in domains facilitates interaction with the constant domains of IgA. Secretory component also becomes disulphide-linked to one of the monomers of dimeric IgA [90], A recent model suggests that it is a cysteine on the first domain of secretory component which links to the unpaired cysteine of the Ca2 domain [77], Fig. 13(b) shows a schematic representation of dimeric secretory IgA. In contrast to serum IgA, secretory IgA shows roughly equal proportions of the two subclasses. 

Although there are two known immunoglobulin transporters, the poly Ig receptor for secretory IgA (14) and the FcRn receptor for IgG (15,16), they most likely do not play a major role in E25 transport across lung epithelium as evidenced by the data described below. 

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