Tripartite Efflux pumps

Question 1

Structure of AcrAB-TolC

Answer 1

outer-membrane channel TolC trimer , the inner membrane secondary transporter AcrB trimer and the periplasmic AcrA hexamer
In isolation, TolC assumes a ‘closed’ resting state opening must result from direct interactions with th a-helical hairpins of AcrA.

Question 2

Mechanism of AcrAB-TolC

Answer 2

AcrB showed that it exists as a
trimer, consisting of three layers parallel to the membrane: a
transmembrane domain, a porter domain(where the rotating protomers are located) , and a TolC docking domain
bind in A substrate binding pocket in the porter domain of each
protomer, with aromatic (aromatic- aromatic/ hydrophobic interaction) and polar residues(hydrogen bonds). different residues-different binding substrates.
inclined helix from the extrusion protomer fills this gap and sterically blocks exit from the binding pocket. protonation of the tight substrate unwinds helix, opens channel to tolc, extruded via shrinking of binding pcoket.

Question 3

MacAB-TolC

Structure

Answer 3

MacAB–TolC pump is 6:2:3
MacA- conserved Glu ring
MacB has a tightly packed dimer interface in the
TMD and this interface does not appear to have the space required to accommodate a transport substrate.

Question 4

MEchanism of MacAB-TolC

Answer 4

substrates are proposed to enter the interior cavity via an opening in MacB ATP-binding causes closure of the periplasmic domain by mechanotransmission. The reduction in interior cavity volume forces the contents of the MacB periplasmic domain, through the MacA
gate ring,

Once the gate ring relaxes to its closed resting state,preventing backflow of substrates. ATP hydrolysis then resets the system with further substrates and the solvent necessary to refill the cavity entering from the periplasm as the interior cavity expands.