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Membrane transport in cells is a fundamental
biological process that is mediated by various channels and
transproter proteins. A major type of such proteins are the
secondary active membrane transporters, which use a solute gradient
to drive the translocation of other substrates. The
glycerol-3-phosphate (G3P) transporter, GlpT, from Escherichia coli
mediates G3P and inorganic phosphate exchange across the bacterial
inner membrane. It possesses 12 transmembrane α-helices
and is a member of the Major Facilitator Superfamily, the largest
secondary transporter family known in the genomes sequenced to date.
In the cell membrane, these proteins are responsible for the
transport of a wide range of solutes, including sugars, amino acids,
neurotransmitters, ions, and toxins.
For more information see
Biochemistry Online: Chapter
6A - Passive and Facilitated Diffusion
II. General Structure
H1 and H7
H2 and H8
H3 and H9
H4 and H10
H5 and H11
H6 and H12
The protein has 12 transmembrane a-helices which can be divided into two similar domains by a pseudo two-fold symmetry.
This pseudo symmetry extends to all helices with H1 related to H7, H2 to H8, et cetera.
Display a-helices with fill
This space filling representation shows the helices in a space filled way.
By rotating the molecule with your mouse, you can see that the molecular pore (which extends through the center of the protein) is closed on one side and open on the other.
Display Periplasmic Residues
The periplasmic side of the molecule is flat and protrudes only slightly into the periplasm and the pore is closed.
This barrier is created by portions of H1 and H7. The space between H1 and H7, is filled with nine aromatic side chains which help close the pore completely.
Display H1 and H7 Closing Region
Display Cytoplasmic Residues
The cytoplasmic residues include the N- and C- termini which also means that
the opening of the pore is within the cytoplasm.
Display All Helices Except H1 and H7
The pore opening can be seen by displaying all helices except H1 and H7 and rotating
the molecule such that you are looking directly at the base of the molecule.
A small opening can be seen.
It is believed that the cytoplasmic loop from H10-H11 plays an integral role in helix alignment when the protein is inserted into the membrane.
This is because it contains two lysine residues (Lys378 and Lys379) which have been shown to aid in insertion of the Glut1 protein (Sato et al. J. Biol. Chem. 274m 24721 (1999)).
Display Both Cytoplasmic and Periplasmic Regions
With this representation, it is clearly shown how the molecule inserts itself into the membrane.
The periplasmic and cytoplasmic residues encompass the region of the protein which is membrane bound and the pore is closed.
Display Arg45 and Arg269
It is noted that Arg45 and Arg269 are embedded in H1 and H7 and by X-ray crystallography, the literature reports that the distance between then is 9.9A.
The optimal hydrogen bond length for the phosphate and Arg residues would be 2.9A,
this means that the phosphate ligand must induce a conformation change in the protein to bring the Arg-Arg distance to the optimal dimension.
When this happens, the periplasmic ends of H1 and H7 separate and the cytoplasmic ends come together to close the cytoplasmic side.
This allows for the inorganic phosphate to be released and G3P to be bound and brought back into the cell.
as situated in H1 and H7
The substrate binding sites as located on H1 and H7.
The GlpT protein works in a "rocker-switch" fashion which exposes the binding site, Arg45 and Arg269, to the periplasm, yielding the outward opening conformation of the molecule. Cytoplasmic binding and periplasmic release of inorganic phosphate would allow its replacement in the substrate-binding site by G3P, which is then transferred into the cytoplasm. Substrate binding is proposed to lower the energy barrier between the inward- and outward-facing conformations of GlpT, facilitating their interconversion and allowing the inorganic phosphate gradient to drive G3P transport.