An outer-membrane protein (OMP) was isolated from a clinical strain of . Changes in growth media did not appreciably affect the appearance of this protein in crude outer-membrane preparations examined by SDS-PAGE. However, the proportion of the protein relative to other OMPs was greater in 24-h cultures than in 48-h cultures. The protein could not be readily solubilised by various conventional detergent extraction techniques but treatment of the insoluble material at 100°C with SDS released the protein, as did overnight extraction at 37°C with SDS. This OMP was heat-modifiable, and thus was similar to the OmpA protein of , with a faster mobility on SDS-PAGE when solubilised at 25°C than at 100°C. The critical temperature for conversion was between 80°C and 90°C. Because of the characteristic heat-modifiability, the protein was called HMP-1 (heat modifiable protein-1). Overnight exposure to EDTA or NaCl at 37°C favoured conversion of the 25°C form to the 100°C form. In intact cells, the protein was labelled by a cell-surface radio-iodination procedure, and thus is at least partially exposed at the cell surface. No reactions between the HMP-1 and antibodies to either OmpA or porin were found by Western blot analysis. A OM preparation containing predominantly HMP-1 had pore-forming ability in a liposome assay. This study is the first report of the isolation and characterisation of a heat-modifiable OMP in , and it is the first description of pore-forming activity in a Bacteroides OM fraction.


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