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J Biol Chem. 2016 Jan 22;291(4):1676-91. doi: 10.1074/jbc.M115.684423. Epub 2015 Nov 20.

The Structure of a Type 3 Secretion System (T3SS) Ruler Protein Suggests a Molecular Mechanism for Needle Length Sensing.

Author information

1
From the Department of Biochemistry and Molecular Biology, the Centre for Blood Research, and.
2
the Centre for Microbial Diseases and Immunity Research, Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
3
From the Department of Biochemistry and Molecular Biology.
4
the Centre for Blood Research, and the Centre for Microbial Diseases and Immunity Research, Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
5
From the Department of Biochemistry and Molecular Biology, the Centre for Blood Research, and ncjs@mail.ubc.ca.

Abstract

The type 3 secretion system (T3SS) and the bacterial flagellum are related pathogenicity-associated appendages found at the surface of many disease-causing bacteria. These appendages consist of long tubular structures that protrude away from the bacterial surface to interact with the host cell and/or promote motility. A proposed "ruler" protein tightly regulates the length of both the T3SS and the flagellum, but the molecular basis for this length control has remained poorly characterized and controversial. Using the Pseudomonas aeruginosa T3SS as a model system, we report the first structure of a T3SS ruler protein, revealing a "ball-and-chain" architecture, with a globular C-terminal domain (the ball) preceded by a long intrinsically disordered N-terminal polypeptide chain. The dimensions and stability of the globular domain do not support its potential passage through the inner lumen of the T3SS needle. We further demonstrate that a conserved motif at the N terminus of the ruler protein interacts with the T3SS autoprotease in the cytosolic side. Collectively, these data suggest a potential mechanism for needle length sensing by ruler proteins, whereby upon T3SS needle assembly, the ruler protein's N-terminal end is anchored on the cytosolic side, with the globular domain located on the extracellular end of the growing needle. Sequence analysis of T3SS and flagellar ruler proteins shows that this mechanism is probably conserved across systems.

KEYWORDS:

Pseudomonas aeruginosa (P. aeruginosa); bacterial pathogenesis; macromolecular assembly; molecular ruler; protein structure; type III secretion system (T3SS); x-ray crystallography

PMID:
26589798
PMCID:
PMC4722450
DOI:
10.1074/jbc.M115.684423
[Indexed for MEDLINE]
Free PMC Article

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