Surface lipoprotein sorting by crosstalk between Lpt and Lol pathways in gram-negative bacteria

Area of Science:

• Bacteriology
• Structural Biology
• Molecular Microbiology

Background:

• Gram-negative bacteria possess an outer membrane (OM) crucial for their structure and pathogenicity.
• Lipopolysaccharide (LPS) and lipoproteins are key OM components, with LPS translocation mediated by LptDE.
• The mechanism of lipoprotein sorting to the cell surface remains largely unknown.

Purpose of the Study:

• To investigate the potential role of LptDE in transporting lipoproteins to the bacterial cell surface.
• To elucidate the structural basis of lipoprotein-LptDE interaction.
• To explore the functional overlap between Lpt and Lol pathways in lipoprotein sorting.

Main Methods:

• Crystal structure determination of LptDE from Pseudomonas aeruginosa and its complex with Escherichia coli lipoprotein LptM.
• Native mass spectrometry.
• Immunofluorescence assays.
• Photocrosslinking assays.

Main Results:

• The crystal structure of the paLptDE-LptM complex reveals that LptM may translocate through LptDE to the OM.
• The LptDE structure suggests a transport mechanism where the protein moiety passes through the β-barrel and acyl chains are externalized.
• Experimental evidence supports a model where lipoproteins are loaded into LptA by LolCDE before LptB2FGCADE complex assembly.

Conclusions:

• LptDE facilitates the transport of surface-exposed lipoproteins (SLPs) to the outer membrane, mirroring LPS transport.
• A potential SLP transport mechanism involving LptA, LolCDE, and the Lpt complex is proposed.
• This study provides evidence for functional interplay between the Lpt and Lol pathways, expanding understanding of OM protein sorting.