Localization of the germination protein GerD to the inner membrane in Bacillus subtilis spores

GerD of Bacillus subtilis is a protein essential for normal spore germination with either L-alanine or a mixture of L-asparagine, D-glucose, D-fructose, and potassium ions. GerD's amino acid sequence suggests that it may be a lipoprotein, indicating a likely location in a membrane. Location in the spore's outer membrane seems unlikely, since removal of this membrane does not result in a gerD spore germination phenotype, suggesting that GerD is likely in the spore's inner membrane. In order to localize GerD within spores, FLAG-tagged GerD constructs were made, found to be functional in spore germination, and detected in immunoblots of spore extracts as not only monomers but also dimers and trimers. Upon fractionation of spore extracts, GerD-FLAG was found in the inner membrane fraction from dormant spores and was present at approximately 2,000 molecules/spore. GerD-FLAG in the inner membrane fraction was solubilized by Triton X-100, suggesting that GerD is a lipoprotein, and the protein was also solubilized by 0.5 M NaCl. GerD-FLAG was not processed proteolytically in a B. subtilis strain lacking gerF (lgt), which encodes prelipoprotein diacylglycerol transferase (Lgt), indicating that when GerD does not have a diacylglycerol moiety, signal sequence processing does not occur. However, unprocessed GerD-FLAG still gave bands corresponding to monomers and dimers of slightly higher molecular weight than that of GerD-FLAG from a strain with Lgt, further suggesting that GerD is a lipoprotein. Upon spore germination, much GerD became soluble and then appeared to be degraded as the germinated spores outgrew and initiated vegetative growth. All of these results suggest that GerD is a lipoprotein associated with the dormant spore's inner membrane that may be released in some fashion from this membrane upon spore germination.