EPR on type I and type II ABC importers

The ubiquitous ABC-transporter family is involved in the ATP-dependent uptake or export of a large variety of substrates. Despite clear structural similarities, evidences for considerable mechanistic diversity start to accumulate within the importers subfamily. We use site directed spin labeling EPR to unravel molecular details of two different types of importers. The ABC type I maltose import system of E.coli is biochemically as well as structurally well characterized [1]. The aim of this study is to use EPR to unveil the details of the interaction between MalE and the transporter MalFGK2 during the nucleotide cycle in response to maltose. In the absence of maltose MalE interacts with the P2-loop of the transporter in the apo-state mostly via the N-lobe, whereas in the presence of maltose the interaction involves both lobes of MalE. In contrast, the conformation of the complex in the ATP-state is only driven by binding of nucleotides, regardless of maltose. The data highlights the importance of the substrate in the mechanism of transport. The translocation mechanism of the ABC type II vitamin B12 import system of E.coli is still unknown [2]. We present here a detailed study of the gating mechanism of the BtuCD-F importer. The translocation gates of BtuCD-F undergo conformational changes in line with a two-state alternating-access model. Binding of ATP drives the gates to an inward-facing conformation, in contrast to the better characterized type I importers. In the presence of ATP, an excess of vitamin B12 promotes the re-opening of the gates towards the periplasm and the dislodgment of BtuF from the transporter. The EPR data allow a productive translocation cycle of the vitamin B12 transporter to be modeled [3].