GLPF: A Structural Variant of the Aquaporin Tetramer

The glycerol uptake facilitator of E. coli (GlpF) (Boos et al., 1990) is one of the few known diffusion facilitators in the inner membrane of this bacterium. Glycerol diffuses into the cell through GlpF and is phosphorylated by the glycerol kinase (GlpK), which prevents back-diffusion. Besides glycerol transport, the diffusion of polyols and urea derivatives through GlpF have been reported (Maurel et al., 1994), but none of these substrates are transported in a phosphorylated state. In contrast, it remains unclear whether GlpF allows the passage of water.