Updated publication reference for PubMed record(s): 31113901. In Gram-negative bacteria, such as Escherichia coli, the heteropentomeric -barrel assembly machine (Bam) folds and inserts proteins into the outer membrane of the cell envelope. Here, we show that the conditional lethal phenotype of a mutant lacking two of the three nonessential lipoproteins, BamB and BamE, is caused by the lethal jamming of the stripped down Bam complex by a normally surface-exposed lipoprotein, RcsF. Our study highlights the importance of the nonessential Bam complex lipoproteins, BamB and BamE, in regulating the interaction between the essential Bam proteins, BamA and BamD and expands our understanding of the role of the Bam complex in outer membrane biogenesis.