The non-®mbrial adhesins, YadA of enteropathogenic Yersinia species, and UspA1 and UspA2 of Moraxella catarrhalis, are established pathogenicity factors. In electron micrographs, both surface proteins appear as distinct`lollipop'-shaped structures forming a novel type of surface projection on the outer membranes. These structures, amino acid sequence analysis of these molecules and yadA gene manipulation suggest a tripartite organization: an N-terminal oval head domain is followed by a putative coiled-coil rod and terminated by a C-terminal membrane anchor domain. In YadA, the head domain is involved in autoagglutination and binding to host cells and collagen. Analysis of the coiled-coil segment of YadA revealed unusual pentadecad repeats with a periodicity of 3.75, which differs signi®cantly from the 3.5 periodicity found in the Moraxella UspAs and other canonical coiled coils. These ®ndings predict that the surface projections are formed by oligomers containing right-(Yersinia) or left-handed (Moraxella) coiled coils. Strikingly, sequence comparison revealed that related proteins are found in many proteobacteria, both human pathogenic and environmental species, suggesting a common role in adaptation to speci®c ecological niches.