Stochastic steps in secondary active sugar transport

Area of Science:

• Biochemistry and structural biology
• Membrane transport mechanisms
• Molecular dynamics simulations

Background:

• Secondary active transporters utilize ion gradients to move molecules across membranes.
• Understanding the dynamic coupling of ion/substrate binding and conformational changes is crucial for transporter function.
• Static protein structures do not reveal the dynamic nature of ion/substrate release events.

Purpose of the Study:

• To investigate the order of ion and substrate release from the inward-facing state of secondary active transporters.
• To elucidate the dynamic mechanisms underlying transporter function at an atomistic level.
• To reconcile conflicting findings regarding the cytoplasmic release of ions and substrates.

Main Methods:

• Molecular simulations of the sugar transporter vSGLT.
• Functional experiments on the human homolog, hSGLT1.
• Analysis of ion and substrate binding and release dynamics.

Main Results:

• Simulations revealed stochastic, unordered release of substrate and ions from the inward-facing state of vSGLT.
• Experimental data confirmed unordered cytoplasmic release for hSGLT1.
• Ordered binding of substrate and ions from the extracellular space was confirmed for hSGLT1.

Conclusions:

• Cytoplasmic release of ions and substrates from secondary active transporters is not ordered.
• Extracellular binding of ions and substrates is an ordered process.
• This lack of coordination in the inward-facing state may be common in the transporter superfamily.