Assembly of the secretion pores GspD, Wza and CsgG into bacterial outer membranes does not require the Omp85 proteins BamA or TamA

• Published in: Molecular microbiology 2015-08, Vol.97 (4), p.616-629
• Main Authors: Dunstan, Rhys A., Hay, Iain D., Wilksch, Jonathan J., Schittenhelm, Ralf B., Purcell, Anthony W., Clark, Joan, Costin, Adam, Ramm, Georg, Strugnell, Richard A., Lithgow, Trevor
• Format: Article
• Language: English
• Subjects: Bacterial Outer Membrane Proteins - chemistry; Bacterial Outer Membrane Proteins - genetics; Bacterial Outer Membrane Proteins - metabolism; Bacterial Secretion Systems - metabolism; Bioassays; Cryoelectron Microscopy; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - genetics; Escherichia coli Proteins - metabolism; Gram-negative bacteria; Gram-Negative Bacteria - genetics; Gram-Negative Bacteria - metabolism; Lipoproteins - chemistry; Lipoproteins - genetics; Lipoproteins - metabolism; Membranes; Microscopy; Porins - chemistry; Porins - genetics; Porins - metabolism; Protein Structure, Secondary; Proteins
• ISSN: 0950-382X; 1365-2958
• DOI: 10.1111/mmi.13055

Summary In Gram‐negative bacteria, β‐barrel proteins are integrated into the outer membrane by the β‐barrel assembly machinery, with key components of the machinery being the Omp85 family members BamA and TamA. Recent crystal structures and cryo‐electron microscopy show a diverse set of secretion pores in Gram‐negative bacteria, with α‐helix (Wza and GspD) or β‐strand (CsgG) transmembrane segments in the outer membrane. We developed assays to measure the assembly of three distinct secretion pores that mediate protein (GspD), curli fibre (CsgG) and capsular polysaccharide (Wza) secretion by bacteria and show that depletion of BamA and TamA does not diminish the assembly of Wza, GspD or CsgG. Like the well characterised pilotins for GspD and other secretins, small periplasmic proteins enhance the assembly of the CsgG β‐barrel. We discuss a model for integral protein assembly into the bacterial outer membrane, focusing on the commonalities and differences in the assembly of Wza, GspD and CsgG. In Gram‐negative bacteria, BamA and TamA assemble β‐barrel proteins into the outer membrane. But other integral membrane proteins with α‐helix (Wza, GspD) or β‐strand (CsgG) transmembrane segments are present in bacterial outer membranes. New assays show that Wza, GspD and CsgG are integrated into the outer membrane without assistance from BamA or TamA.