Assembling of AcrB trimer in cell membrane.

Many membrane proteins exist and function as oligomers, but how monomers oligomerize in the cell membrane remains poorly understood. AcrB is an obligate homo-trimer. We previously found that the folding of individual subunit precedes oligomerization. Following folding, individual AcrB subunits must locate and interact with each other in order to dimerize and eventually trimerize. It has been unclear if AcrB trimerization is a spontaneous process following the "chance encounter and random assembling" mechanism. In other words, it is currently unknown whether monomeric subunits diffuse freely to "search" for each other after they are co-translationally inserted and folded into the cell membrane. Using four sets of experiments exploiting AcrB variants with different fusion tags, disulfide trapping, and activity measurement, here we showed that AcrB variants co-expressed in the same Escherichia coli cell did co-assemble into hybrid trimers in vivo. However, the level of co-assembly measured experimentally was not consistent with calculations derived from random assembling. The potential role of the polysome structure during protein translation and the resultant clustering effect were discussed as a potential explanation for the observed bias in AcrB subunit assembling in vivo. Our results provide new insights into the dynamic assembling and equilibration process of obligate homo-oligomeric membrane proteins in the cell membrane.