Exploring conformational equilibria of a heterodimeric ABC transporter

Area of Science:

• Biochemistry
• Structural Biology
• Membrane Transport

Background:

• ABC exporters facilitate substrate transport across membranes.
• They couple ATP hydrolysis to conformational changes in transmembrane domains (TMDs) and nucleotide-binding domains (NBDs).
• TM287/288 is a heterodimeric ABC exporter with a unique ATP-binding site.

Purpose of the Study:

• To investigate the conformational dynamics of the heterodimeric ABC exporter TM287/288.
• To elucidate the role of nucleotide binding and hydrolysis in transporter function.
• To understand the structural differences between heterodimeric and homodimeric ABC exporters.

Main Methods:

• Double Electron-Electron Resonance (DEER) measurements were employed.
• Experiments were conducted on the TM287/288 transporter from *Thermotoga maritima*.
• The study examined transporter behavior under varying nucleotide conditions and temperatures.

Main Results:

• TM287/288 exists in an inward-facing/outward-facing (IF/OF) equilibrium influenced by nucleotides.
• ATP binding alone partially shifts the transporter to the OF state.
• Nucleotide trapping (pre- or post-hydrolysis) is crucial for significant conformational changes.
• At high temperatures without nucleotides, NBDs disengage asymmetrically, while TMDs remain stable.
• Nucleotide binding at the degenerate site inhibits complete NBD separation, distinguishing it from homodimeric exporters.
• Hydrolysis-independent NBD dimer closure occurs and is stabilized upon nucleotide hydrolysis commitment.

Conclusions:

• Nucleotide binding and hydrolysis play distinct roles in regulating ABC exporter conformation.
• Hydrolysis-independent NBD dimer closure is a key feature of heterodimeric ABC exporters.
• These findings provide insights into the mechanism of substrate transport by ABC transporters.