Characterization of a major envelope protein from the rumen anaerobe Selenomonas ruminantium OB268

Cell envelopes from the Gram-negative staining but phylogenetically Gram-po sitive rumen anaerobe Selenomonas ruminantium OB268 contained a major 42 kD a heat modifiable protein. A similarly sized protein was present in the env elopes of Selenomonas ruminantium D1 and Selenomonas infelix. Sodium dodecy l sulfate polyacrylamide gel electrophoresis of Triton X-100 extracted cell envelopes from S. ruminantium OB268 showed that they consisted primarily o f the 42 kDa protein. Polyclonal antisera produced against these envelopes cross-reacted only with the 42 kDa major envelope proteins in both S. rumin antium D1 and S. infelix, indicating a conservation of antigenic structure among each of the major envelope proteins. The N-terminus of the 42 kDa S. ruminantium OB268 envelope protein shared significant homology with the S-l ayer (surface) protein from Thermus thermophilus, as well as additional env elope proteins containing the cell surface binding region known as a surfac e layer-like homologous (SLH) domain. Thin section analysis of Triton X-100 extracted envelopes demonstrated the presence of an outer bilayer overlayi ng the cell wall, and a regularly ordered array was visible following freez e-fracture etching through this bilayer. These findings suggest that the re gularly ordered array may be composed of the 42 kDa major envelope protein. The 42 kDa protein has similarities with regularly ordered outer membrane proteins (rOMP) reported in certain Gram-negative and ancient eubacteria.