The T-cell-stimulating 17-kilodalton protein of Francisella tularensis LVS is a lipoprotein

Abstract

A T-cell-stimulating, membrane-located 17-kDa protein of the live vaccine strain Francisella tularensis LVS has previously been cloned and sequenced. In the present study, it is shown to be a lipoprotein. When F. tularensis was grown in the presence of [3H]palmitate, several proteins of the organism, including a 17-kDa protein, were radiolabeled. The labeled 17-kDa protein was found by Western blot (immunoblot) analysis to be identical to the cloned protein. It was located in the detergent phase after partitioning with the nonionic detergent Triton X-114, thereby behaving like a hydrophobic integral membrane protein. The protein was predominantly hydrophilic and contained no putative transmembrane domain. The presence of fatty acids is therefore the probable explanation of the membrane location of the 17-kDa protein. The amino acid sequence of the 17-kDa protein contains the tetrapeptide Leu-Ala-Ser-Cys, which is a recognition sequence of the lipoprotein signal peptidase. Globomycin, a specific inhibitor of the peptidase, inhibited maturation of the 17-kDa lipoprotein. The protein incorporated [3H]palmitate also when expressed by Escherichia coli. The 17-kDa lipoprotein was recognized not only by T cells but also by serum antibodies of F. tularensis-primed individuals.