Part of the outer membrane protein assembly complex (Bam), which is involved in assembly and insertion of beta-barrel proteins into the outer membrane. Nonessential member of the complex, which may orient the flexible periplasmic domain of BamA for interaction with other Bam components, chaperones and nascent outer membrane proteins. Efficient substrate folding and insertion into the outer membrane requires all 5 subunits (PubMed:20378773, PubMed:21823654, PubMed:27686148). A lateral gate may open between the first and last strands of the BamA beta-barrel that allows substrate to insert into the outer membrane; comparison of the structures of complete and nearly complete Bam complexes show there is considerable movement of all 5 proteins (PubMed:27686148, PubMed:26900875, PubMed:26901871). Part of the Bam complex, which is composed of the outer membrane protein BamA, and four lipoproteins BamB, BamC, BamD and BamE. Monomer. Interacts directly with BamA. The Bam complex has the shape of a hat, with the BamA beta-barrel crown in the outer membrane and the periplasmic brim formed by the BamA POTRA domains and the 4 lipoproteins (PubMed:27686148, PubMed:26900875, PubMed:26901871). With 3 palmitic acid (C16) acyl chains. With 2 palmitic (C16) and 1 stearic (C18) acid acyl chains. Belongs to the BamB family.