Localization of a germinant receptor protein (GerBA) to the inner membraneof Bacillus subtilis spores

Dormant Bacillus subtilis spores germinate in response to specific nutrient s called germinants, which are recognized by multisubunit receptor complexe s encoded by members of the gerA family of operons, of which the gerB opero n is a member, The germinant receptors are expected to be membrane associat ed, but there is some debate about whether they are located in the inner or outer spore membrane. In this study we have used Western blot analysis to determine the precise location of GerBA, a gerB-encoded receptor protein, i n various spore fractions. GerBA was not extracted from spores by a decoati ng treatment that removes the coat and outer membrane but was present in ly sates from decoated spores and in the insoluble fraction (termed P100) from such lysates that contained inner-membrane vesicles. GerBA was also solubi lized from the P100 fraction with detergent but not with high salt. These f indings suggest that GerBA is an integral membrane protein located in the s pore's inner membrane. Consistent with this idea, GerBA was present in the cell membrane of the outgrowing spore, a membrane that is derived from the dormant spore's inner membrane. Based on these observations we propose that GerBA and probably the entire GerB germinant receptor are located in the i nner membrane of the dormant spore. We also estimated that there are only 2 4 to 40 molecules of GerBA per spore, a number that is consistent with the previously reported low level of gerB operon expression and with the putati ve receptor function of the proteins encoded by the gerB operon.