Journal
BIOCHEMICAL SOCIETY TRANSACTIONS
Volume 39, Issue -, Pages 751-760Publisher
PORTLAND PRESS LTD
DOI: 10.1042/BST0390751
Keywords
ATP-binding cassette transporter (ABC transporter); folding; membrane protein; subunit interaction; unfolding
Categories
Funding
- Biotechnology and Biological Sciences Research Council
- European Union via the European Membrane Protein Consortium (EMeP)
- Royal Society
- BBSRC [BB/G002037/1] Funding Source: UKRI
- Biotechnology and Biological Sciences Research Council [BB/G002037/1] Funding Source: researchfish
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Prokaryotic importers from the large family of ABC (ATP-binding cassette) transporters comprise four separate subunits: two membrane-embedded and two cytoplasmic ATP-binding subunits. This modular construction makes them ideal candidates for studies of the intersubunit interactions of membrane protein complexes that contain both hydrophobic and hydrophilic subunits. In the present paper, we focus on the vitamin B-12 importer of Escherichia coli, BtuCD, that contains two transmembrane BtuC subunits and two ATP-binding BtuD subunits. We have studied the factors that induce subunit dissociation and unfolding in vitro. The BtuCD complex remains intact in alcohol and mild detergents, but urea or SDS separate the BtuC and BtuD subunits, with 6 M urea causing 80% of BtuD to be removed from BtuCD. ATP is found to stabilize the complex as a result of its binding to the BtuD subunits. In the absence of ATP, low concentrations of urea (0.5-3 M) also induce some unfolding, with approximately 14% reduction in helicity in 3 M urea, whereas, in the presence of ATP, no changes are observed. Disassembly at the BtuD-BtuD dimeric interface in BtuCD can be achieved with smaller concentrations of urea (0.5-3 M) than that required to cause disassembly at the BtuC-BtuD transmission interface (3-8 M), suggesting a stronger interaction of the latter. The results also suggest that unfolding and disassociation of subunits appear to be coupled processes. Our work provides insights into the subunit interactions of an ABC transporter and lays the foundation for studies of the reassembly of BtuCD.
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