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CeNS Colloquium

Kleiner Physikhörsaal, LMU
Date: 20.04.2018, Time: 15:30h

How do ABC transporters convert the energy of ATP to drive conformational change?

Dr. Thomas Stockner, Medizinische Universität Wien

The human genome codes for 42 ABC membrane transporters and 4 ABC proteins, grouped into the subfamilies A-G. They have in common to use ATP binding and hydrolysis to energize the function and the associated conformational changes. The ABC membrane transporter have been intensively studied, the appearing of crystal and cryoEM structures in the last 12 years have provided much needed structural insight to understand the function of these molecular machines. These structures have not unambiguously shown the mechanism of how the energy of ATP is harvested and converted into conformational changes that lead to directional transport across the membrane. I will show data on the energetic profiles of conformational changes of the nucleotide binding domains (NBDs) and use these to elaborate on motions that ATP binding and hydrolysis can induce in the NBDs.

A significant number of ABC exporters carry a degenerate nucleotide binding site (NBS), which shows a strongly reduced ATPase activity. The hallmark of the degenerate NBS, which is typically NBS1, is the substitution of the catalytic glutamate in the Walker B motif. I will show data an the highly homologuous transporters ABCB1 and ABCB11 to elucidate aspects  of this process. The multidrug transporter ABCB1 (P-glycoprotein) and the bile salt export pump ABCB11 (BSEP) are both full-length members of the ABCB subfamily and share 49% sequence identity, but importantly,  ABCB1 has two canonical NBSs, while in ABCB11 one is degenerate. A comparison of their NBD interfaces reveals that the entire interface differs in only four residues, which are all located in NBS1. When introducing the analogous catalytic glutamate mutation (E556M) into ABCB1, the transporter becomes transport incompetent and conformationally restrained. However, when including the additional three mutations, the transporter regains the ability to hydrolyse ATP and undergo conformational changes also in the TMDs, which are reminiscent of wild type ABCB1. The data indicate that the non-canonical NBS1, as present in ABCB11, has the ability to escape the conformationally locked state of the single catalytic glutamate mutant (E556M) by avoiding ATP occlusion.