The translocation assembly module (TAM) catalyzes the assembly of bacterial outer membrane proteins in vitro

Abstract

The translocation and assembly module (TAM) has been proposed to play a crucial role in the assembly of a small subset of outer membrane proteins (OMPs) in Proteobacteria based on experiments conducted in vivo using tamA and tamB mutant strains and in vitro using biophysical methods. TAM consists of an OMP (TamA) and a periplasmic protein that is anchored to the inner membrane by a single α helix (TamB). Here we examine the function of the purified E. coli complex in vitro after reconstituting it into proteoliposomes. We find that TAM catalyzes the assembly of four model OMPs nearly as well as the β-barrel assembly machine (BAM), a universal heterooligomer that contains a TamA homolog (BamA) and that catalyzes the assembly of almost all E. coli OMPs. Consistent with previous results, both TamA and TamB are required for significant TAM activity. Our study provides direct evidence that TAM can function as an independent OMP insertase and describes a new method to gain insights into TAM function.

The bacterial TAM complex has been proposed to participate in the assembly of some outer membrane proteins (OMPs) based primarily on in vivo experiments that used mutant strains. Here, Wang et al. use the purified complex reconstituted into proteoliposomes to demonstrate that TAM can indeed catalyze OMP assembly in vitro.