The oligomeric structure of the plasmid-encoded outer membrane protein YadA of Yersinia enterocolitica was studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and sucrose gradient sedimentation, respectively. The apparent molecular weight (M(r)) of the oligomeric 200-kDa YadA species detected by SDS-PAGE varied from 152,000 to 240,000 depending on the respective acrylamide concentration. The atypical electrophoretic behavior of the 200-kDa YadA species results form an exceptionally high relative free mobility as revealed by the Ferguson plot. In contrast, the apparent M(r) of 53,000 of the YadA monomer was independent of the acrylamide concentration. An additional oligomeric 116-kDa YadA species was detected by SDS-PAGE when membrane preparations of Y. enterocolitica were solubilized in SDS at 37 degrees C. The gel-purified 116-kDa YadA species was completely converted to the 200-kDa species by heating at 100 degrees C and to the monomeric form (M(r) 53,000) by heating in the presence of 10 M urea without reducing agents, respectively. This suggests that the 116-kDa YadA species represents the native oligomeric form of YadA, whereas the 200-kDa species is only generated from native YadA during denaturation in SDS. The significance of the 116-kDa YadA species is also supported by the rather slow sedimentation at about 6 S of detergent-solubilized YadA in sucrose gradients, which probably contains only two or three monomers.