The glucose permease of Bacillus subtilis is a single polypeptide chain that functions to energize the sucrose permease.

Biochemical, immunological, and sequence analyses demonstrated that the glucose permease of Bacillus subtilis, the glucose-specific Enzyme II of the phosphoenolpyruvate-dependent phosphotransferase system, is a single polypeptide chain with a C-terminal Enzyme III-like domain. A flexible hydrophilic linker, similar in length and amino acid composition to linkers previously identified in other regulatory or sensory transducing proteins, functions to tether the Enzyme IIIGlc-like domain of the protein to the membrane-embedded Enzyme IIGlc. Evidence is presented demonstrating that the Enzyme IIIGlc-like domain of the glucose permease plays a dual role and functions in the transport and phosphorylation of both glucose and sucrose. The sucrose permease appears to lack a sucrose-specific Enzyme III-like domain or a separate, soluble IIIScr protein. Enzyme IIScr was capable of utilizing the IIIGlc-like domain of the glucose permease regardless of whether the IIIGlc polypeptide was provided as a purified, soluble protein, as a membrane-bound protein within the same membrane as Enzyme IIScr, or as a membrane-bound protein within membrane fragments different from those bearing Enzyme IIScr. These observations suggest that the IIIGlc-like domain is an autonomous structural unit that assumes a conformation independent of the hydrophobic, N-terminal intramembranal domain of Enzyme IIGlc. Preferential uptake and phosphorylation of glucose over sucrose has been demonstrated by both in vivo transport studies and in vitro phosphorylation assays. Addition of the purified IIIGlc-like domain strongly stimulated the phosphorylation of sucrose, but not that of glucose, in phosphorylation assays that contained the two sugars simultaneously. The results suggest that the preferential uptake of glucose over sucrose is determined by competition of the corresponding sugar-specific permeases for the common P approximately IIIGlc/Scr domain.