Abstract
The β-barrel assembly machinery (BAM) complex of Escherichia coli is a multiprotein machine that catalyzes the essential process of assembling outer membrane proteins. The BAM complex consists of five proteins: one membrane protein, BamA, and four lipoproteins, BamB, BamC, BamD, and BamE. Here, we report the first crystal structure of a Bam lipoprotein complex: the essential lipoprotein BamD in complex with the N-terminal half of BamC (BamC(UN) (Asp(28)-Ala(217)), a 73-residue-long unstructured region followed by the N-terminal domain). The BamCD complex is stabilized predominantly by various hydrogen bonds and salt bridges formed between BamD and the N-terminal unstructured region of BamC. Sequence and molecular surface analyses revealed that many of the conserved residues in both proteins are found at the BamC-BamD interface. A series of truncation mutagenesis and analytical gel filtration chromatography experiments confirmed that the unstructured region of BamC is essential for stabilizing the BamCD complex structure. The unstructured N terminus of BamC interacts with the proposed substrate-binding pocket of BamD, suggesting that this region of BamC may play a regulatory role in outer membrane protein biogenesis.
Highlights
The Escherichia coli -barrel assembly machinery complex (BamABCDE) facilitates outer membrane protein assembly
The molar mass of the complex was verified by multiangle light-scattering analysis (60.1 Ϯ 1.8 kDa), which is consistent with the sum of the calculated molecular masses (64.4 kDa) of the BamC (36.4 kDa) and BamD (28.0 kDa) constructs used in this study
The truncated forms of BamC created for this study are as follows: 1) BamCN, 2) BamCC, 3) BamCNC, and 4) BamCUN
Summary
The Escherichia coli -barrel assembly machinery complex (BamABCDE) facilitates outer membrane protein assembly. We report the first crystal structure of a Bam lipoprotein complex: the essential lipoprotein BamD in complex with the N-terminal half of BamC (BamCUN (Asp28–Ala217), a 73-residue-long unstructured region followed by the N-terminal domain).
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