Previously, we have shown that gp330, the putative autoantigen of Heymann nephritis which is located on the glomerular and tubular epithelial cells, is a receptor for plasminogen. During our analysis of the function of gp330, we also observed by means of Western analysis the binding of gp330 to a 45-kDa protein. This 45-kDa protein was obtained from extracts of rat kidney and purified to apparent homogeneity. Enzyme-linked immunosorbent assay analysis indicate that gp330 can bind to the 45-kDa protein. In fact, both the 45-kDa protein and plasminogen can bind to gp330 simultaneously, indicating that gp330 has two separate sites for each respective protein. No inhibition of binding of either ligand to gp330 is observed when both ligands were simultaneously incubated with immobilized gp330. A rabbit polyclonal antibody raised against this 45-kDa protein was used to screen a rat kidney lambda gt11 expression library. Sequence analysis of the isolated clones revealed that this protein is similar to the human alpha 2-macroglobulin receptor-related protein and murine heparin-binding protein-44. It is also identical to a previously reported, supposedly pathogenic, domain of gp330. However, our results show that the cDNAs encode for a 45-kDa protein, which is not a part of gp330 but a separate protein which binds to gp330.