Which of the following is NOT true about the substrate-binding protein (SBP) of the ABC transport system?

A) It floats in the periplasm of Gram-negative cells.
B) It is tethered to the cell surface in Gram-positive cells.
C) It binds to the periplasmic face of the channel and releases the solute.
D) It snags the appropriate solute.
E) It is required for both the uptake and efflux systems.