We have recently purified and partially sequenced a microfibrillar protein from human aortic adventitia (aneurysm-associated antigenic protein, 40 kDa [AAAP-40]) that is immunoreactive with immunoglobulin (IgG) from the wall of abdominal aortic aneurysms (AAAs). It shares motifs with Ig kappa (which may act as a binding site for interaction with integrins), cytomegalovirus (which may be a molecular mimic in the pathogenesis of AAA), and vitronectin and the fibrinogens. A cDNA library was constructed from the aortic adventitia of a AAA. The library was screened with either rabbit anti-vitronectin antibody or rabbit anti-fibrinogen antibody. Positive plaques were purified and expressed in a strain of Escherichia coli. The clone sequences were analyzed. The expressed proteins were separated by SDS/PAGE and the immunoblots were probed with either AAA IgG or anti-human Ig kappa antibody. Experimental cell lines, transfected with the clones (clones 1 and 5), synthesized recombinant proteins (rAAAP-CL1 and rAAAP-CL5), detectable in Western immunoblots with AAA IgG. A prediction of the tertiary structure resembles well-characterized cell adhesion molecules. These findings suggest that there is a novel family of matrix proteins that may use immunoglobulin motifs as binding sites for cellular integrins and that there are matrix proteins in addition to AAAP-40 that may serve as autoantigens in the pathogenesis of AAA disease.