Glycerol kinase of Escherichia coli is activated by interaction with the glycerol facilitator

Abstract

Glycerol transport is commonly cited as the only example of facilitated diffusion across the Escherichia coli cytoplasmic membrane. Two proteins, the glycerol facilitator and glycerol kinase, are involved in the entry of external glycerol into cellular metabolism. The glycerol facilitator is thought to act as a carrier or to form a selective pore in the cytoplasmic membrane, whereas the kinase traps the glycerol inside the cell as sn-glycerol-3-phosphate. We found that the kinetics of glycerol uptake in a facilitator-minus strain are significantly different from the kinetics of glycerol uptake in the wild type. Free glycerol was not observed inside wild-type cells transporting glycerol, and diffusion of glycerol across the cytoplasmic membrane was not the rate-limiting step for phosphorylation in facilitator-minus mutants. Therefore, the kinetics of glycerol phosphorylation are different, depending on the presence or absence of the facilitator protein. We conclude that there is an interaction between the glycerol facilitator protein and glycerol kinase that stimulates kinase activity, analogous to the hexokinase- and glycerol kinase-porin interactions in mitochondria.

The 6.9-Å Structure of GlpF: A Basis for Homology Modeling of the Glycerol Channel from Escherichia coli

“…However, studies of the solubilized glycerol channel GlpF from Escherichia coli (Braun et al, 2000) revealed a square-shaped particle similar in size and mass to those found for aquaporins (Hasler et al, 1998;Ringler et al, 1999;Walz et al, 1994). A tetrameric structure of GlpF also supports the model of direct interaction of the tetrameric glycerol kinase from E. coli with GlpF (Voegele et al, 1993).…”

Analysis of the Pore of the Unusual Major Intrinsic Protein Channel, Yeast Fps1p

“…Furthermore GUP1 and GUP2 have recently been proposed to be involved in active transport of glycerol in yeasts (13). In contrast, the physiologically relevant role of GlpF from Escherichia coli, the closest bacterial homolog of Fps1p with 30.5% identity, is likely to be import of glycerol for catabolism (14,15).…”

Secondary solute transport in bacteria

“…The GIpF protein has been described as a cytoplasmic membrane 'channel' which also facilitates the passage of polyhydric alcohols such as ribitol and erythritol as well as unrelated small molecules like urea and glycine [31]. The postulated facilitator function of GIpF has recently been questioned, because passive diffusion of glycerol across the cytoplasmic membrane is not rate-limiting for phosphorylation in a GlpF-mutant [32]. On the basis of these observations it has been suggested that GIpF acts as an activator of glycerol kinase (GlpK), although the role of GIpF in facilitating transport has not been refuted.…”