|Title||Conformational changes that coordinate the activity of BamA and BamD allowing β-barrel assembly.|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||McCabe, AL, Ricci, D, Adetunji, M, Silhavy, TJ|
|Date Published||2017 Jul 31|
Most integral outer membrane proteins (OMPs) of Gram-negative bacteria such as Escherichia coli assume a β-barrel structure. The β-barrel Assembly Machine (Bam)-a five-member complex composed of β-barrel OMP BamA and four associated lipoproteins, BamB,C,D, and E-folds and inserts OMPs into the outer membrane. The two essential proteins, BamA and BamD interact to stabilize two subcomplexes, BamAB and BamCDE and genetic and structural evidence suggests that interactions between BamA and BamD occur via an electrostatic interaction between a conserved aspartate residue in a periplasmic domain of BamA and a conserved arginine in BamD. In this work, we characterize charge-change mutations at these key BamA and BamD residues and nearby charged residues in BamA with respect to OMP assembly and Bam complex stability. We show that Bam complex stability does not correlate with function, that BamA and BamD must adopt at least two active conformational states during OMP assembly, and that these charged residues are not required for function. Rather, these charged residues are important for coordinating the activities of BamA and BamD to allow efficient OMP assembly. We present a model of OMP assembly wherein recognition and binding of unfolded OMP substrate by BamA and BamD induces a signaling interaction between the two proteins causing conformational changes necessary for the assembly reaction to proceed. In analogy to signal sequence recognition by SecYEG, we believe these BamA-BamD interactions ensure both substrate and complex are competent for OMP assembly before the assembly reaction commences.IMPORTANCE Conformational changes in the proteins of the β-barrel Assembly Machine (Bam complex) are associated with the folding and assembly of outer membrane proteins (OMPs) in Gram-negative bacteria. We show that electrostatic interactions between the two essential proteins BamA and BamD coordinate conformational changes upon binding of unfolded substrate that allow the assembly reaction to proceed. Mutations affecting this interaction are lethal not because they destabilize the Bam complex but rather because they disrupt this coordination. Our model of BamA-D interactions regulating conformation in response to proper substrate interaction is reminiscent of conformational changes the Sec machinery undergoes after signal sequence recognition that insure protein quality control.
|Alternate Journal||J. Bacteriol.|