Diverse C-Terminal Sequences Involved in Flavobacterium johnsoniae Protein Secretion

Area of Science:

• Microbiology
• Molecular Biology
• Bacterial Protein Secretion

Background:

• Type IX secretion systems (T9SSs) are crucial for protein export across the outer membrane in Bacteroidetes.
• T9SS substrates typically possess signal peptides and conserved carboxy-terminal domains (CTDs) for secretion.
• CTDs are broadly classified into type A (TIGR04183) and type B (TIGR04131) families.

Purpose of the Study:

• To functionally characterize diverse CTDs involved in protein secretion via the F. johnsoniae T9SS.
• To investigate the cross-phylum functionality of type A CTDs.
• To explore the role of type B CTDs in the secretion pathway.

Main Methods:

• Fusion of F. johnsoniae CTDs to superfolder green fluorescent protein (sfGFP) with an N-terminal signal peptide.
• Assessment of sfGFP secretion across the outer membrane mediated by different CTDs.
• Functional testing of type A CTDs from various Bacteroidetes species in F. johnsoniae.

Main Results:

• F. johnsoniae CTDs from RemA, AmyB, and ChiA (type A) facilitated sfGFP secretion, requiring 80-100 amino acids from the C-terminus.
• Type A CTDs from distantly related Bacteroidetes species were functional in the F. johnsoniae T9SS.
• The F. johnsoniae SprB type B CTD was necessary for its secretion, but failed to mediate sfGFP secretion, suggesting additional requirements.

Conclusions:

• Type A CTDs are conserved and functionally interchangeable across diverse members of the Bacteroidetes phylum for T9SS-mediated secretion.
• Type B CTDs may have more complex requirements or interact differently with the T9SS machinery.
• Understanding CTD function is key to deciphering the T9SS mechanism in bacteria.