The Impact of Three-Dimensional Structure on the Expression of PIA Alloantigens on Human Integrin

We have compared the binding of affinity-purified anti-PI”’ IgG from seven nonrelated donors with chimeric integrin subunit p3 molecules expressed in the baculovirus-Spodoptera frugiperda insect cell system. p3 chimeras were engineered t o include segments of antigenic human p3 sequences spliced t o intervening segments of nonantigenic Xenopusp3 sequence. Our results clearly show that antibodies from all seven donors will bind to nondenatured molecules containing the antigenic human p3 C y ~ * ~ C y s ~ ~ loop only when it is presented in a correct orientation that must be maintained by noncontiguous human sequences. Key downstream sequences are located within the region p3288490, flanking either side of the putative long-range disulfide at Cys435. Although our results confirm unambiguously that the Leu/Pro polymorphism at position 33 in human p3 is necessary for the expression of PI” epitopes, they also indi-