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2015 ; 6
(6
): e01867-15
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gab.com Text
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Structural Insight into How Bacteria Prevent Interference between Multiple
Divergent Type IV Secretion Systems
#MMPMID26646013
Gillespie JJ
; Phan IQ
; Scheib H
; Subramanian S
; Edwards TE
; Lehman SS
; Piitulainen H
; Rahman MS
; Rennoll-Bankert KE
; Staker BL
; Taira S
; Stacy R
; Myler PJ
; Azad AF
; Pulliainen AT
mBio
2015[Dec]; 6
(6
): e01867-15
PMID26646013
show ga
Prokaryotes use type IV secretion systems (T4SSs) to translocate substrates
(e.g., nucleoprotein, DNA, and protein) and/or elaborate surface structures
(i.e., pili or adhesins). Bacterial genomes may encode multiple T4SSs, e.g.,
there are three functionally divergent T4SSs in some Bartonella species (vir,
vbh, and trw). In a unique case, most rickettsial species encode a T4SS (rvh)
enriched with gene duplication. Within single genomes, the evolutionary and
functional implications of cross-system interchangeability of analogous T4SS
protein components remains poorly understood. To lend insight into cross-system
interchangeability, we analyzed the VirB8 family of T4SS channel proteins.
Crystal structures of three VirB8 and two TrwG Bartonella proteins revealed
highly conserved C-terminal periplasmic domain folds and dimerization interfaces,
despite tremendous sequence divergence. This implies remarkable structural
constraints for VirB8 components in the assembly of a functional T4SS. VirB8/TrwG
heterodimers, determined via bacterial two-hybrid assays and molecular modeling,
indicate that differential expression of trw and vir systems is the likely
barrier to VirB8-TrwG interchangeability. We also determined the crystal
structure of Rickettsia typhi RvhB8-II and modeled its coexpressed divergent
paralog RvhB8-I. Remarkably, while RvhB8-I dimerizes and is structurally similar
to other VirB8 proteins, the RvhB8-II dimer interface deviates substantially from
other VirB8 structures, potentially preventing RvhB8-I/RvhB8-II
heterodimerization. For the rvh T4SS, the evolution of divergent VirB8 paralogs
implies a functional diversification that is unknown in other T4SSs.
Collectively, our data identify two different constraints (spatiotemporal for
Bartonella trw and vir T4SSs and structural for rvh T4SSs) that mediate the
functionality of multiple divergent T4SSs within a single bacterium. IMPORTANCE:
Assembly of multiprotein complexes at the right time and at the right cellular
location is a fundamentally important task for any organism. In this respect,
bacteria that express multiple analogous type IV secretion systems (T4SSs), each
composed of around 12 different components, face an overwhelming complexity. Our
work here presents the first structural investigation on factors regulating the
maintenance of multiple T4SSs within a single bacterium. The structural data
imply that the T4SS-expressing bacteria rely on two strategies to prevent
cross-system interchangeability: (i) tight temporal regulation of expression or
(ii) rapid diversification of the T4SS components. T4SSs are ideal drug targets
provided that no analogous counterparts are known from eukaryotes. Drugs
targeting the barriers to cross-system interchangeability (i.e., regulators)
could dysregulate the structural and functional independence of discrete systems,
potentially creating interference that prevents their efficient coordination
throughout bacterial infection.