Hook-basal-body assembly state dictates substrate specificity of the flagellar type-III secretion system

The assembly of the bacterial flagellum is orchestrated by the secretion of distinct early and late secretion substrates via the flagellar-specific type-III secretion system (fT3SS). However, how the fT3SS is able to distinguish between the different (early and late) substrate classes during flagellar assembly remains poorly understood. In this study, we investigated the substrate selectivity and specificity of the fT3SS of Salmonella enterica at different assembly stages. For this, we developed an experimental setup that allowed us to synchronize hook-basal-body assembly and to monitor early and late substrate secretion of fT3SSs operating in either early or late secretion mode, respectively. Our results demonstrate that the fT3SS features a remarkable specificity for only the substrates required at the respective assembly stage. No crosstalk of substrates was observed for fT3SSs operating in the opposing secretion mode. We further found that a substantial fraction of fT3SS surprisingly remained in early secretion mode. Our results thus suggest that the secretion substrate specificity switch of the fT3SS is unidirectional and irreversible. The developed secretion substrate reporter system further provides a platform for future investigations of the underlying molecular mechanisms of the elusive substrate recognition of the T3SS.

Control of membrane barrier during bacterial type-III protein secretion

Type-III secretion systems (T3SSs) of the bacterial flagellum and the evolutionarily related injectisome are capable of translocating proteins with a remarkable speed of several thousand amino acids per second. Here, we investigate how T3SSs are able to transport proteins at such a high rate while preventing the leakage of small molecules. Our mutational and evolutionary analyses demonstrate that an ensemble of conserved methionine residues at the cytoplasmic side of the T3SS channel create a deformable gasket (M-gasket) around fast-moving substrates undergoing export. The unique physicochemical features of the M-gasket are crucial to preserve the membrane barrier, to accommodate local conformational changes during active secretion, and to maintain stability of the secretion pore in cooperation with a plug domain (R-plug) and a network of salt-bridges. The conservation of the M-gasket, R-plug, and salt-bridge network suggests a universal mechanism by which the membrane integrity is maintained during high-speed protein translocation in all T3SSs.

Type-III secretion systems (T3SSs) are capable of translocating proteins with high speed while maintaining the membrane barrier for small molecules. Here, a structure-function analysis of the T3SS pore complex elucidates the precise mechanisms enabling the gating and the conformational changes required for protein substrate secretion.

A flagellum-specific chaperone facilitates assembly of the core type III export apparatus of the bacterial flagellum

Many bacteria move using a complex, self-assembling nanomachine, the bacterial flagellum. Biosynthesis of the flagellum depends on a flagellar-specific type III secretion system (T3SS), a protein export machine homologous to the export machinery of the virulence-associated injectisome. Six cytoplasmic (FliH/I/J/G/M/N) and seven integral-membrane proteins (FlhA/B FliF/O/P/Q/R) form the flagellar basal body and are involved in the transport of flagellar building blocks across the inner membrane in a proton motive force-dependent manner. However, how the large, multi-component transmembrane export gate complex assembles in a coordinated manner remains enigmatic. Specific for most flagellar T3SSs is the presence of FliO, a small bitopic membrane protein with a large cytoplasmic domain. The function of FliO is unknown, but homologs of FliO are found in >80% of all flagellated bacteria. Here, we demonstrate that FliO protects FliP from proteolytic degradation and promotes the formation of a stable FliP–FliR complex required for the assembly of a functional core export apparatus. We further reveal the subcellular localization of FliO by super-resolution microscopy and show that FliO is not part of the assembled flagellar basal body. In summary, our results suggest that FliO functions as a novel, flagellar T3SS-specific chaperone, which facilitates quality control and productive assembly of the core T3SS export machinery.

Many bacteria use the bacterial flagellum for directed movement in various environments. The assembly and function of the bacterial flagellum and the related virulence-associated injectisome relies on protein export via a conserved type III secretion system (T3SS). The multicomponent transmembrane core export apparatus of the flagellar T3SS consists of FlhA/B and FliP/Q/R and must assemble in a highly coordinated manner. In the present study, we determined the role of the transmembrane protein FliO in the maturation of the flagellar core protein export apparatus. We show that FliO functions as a flagellum-specific chaperone during the initial step of export apparatus assembly. FliO facilitates the efficient formation of a stable FliP–FliR core complex and is thus required for quality management and productive assembly of the flagellar export apparatus. Our results suggest a coordinated assembly process of the flagellar core export apparatus that nucleates with the FliO-dependent formation of a FliP–FliR complex. Subsequent incorporation of FliQ, FlhB, and FlhA leads to the assembly of a secretion-competent flagellar T3SS.

Functional knockdown of VCAM-1 at the posttranslational level with ER retained antibodies

Vascular cell adhesion molecule 1 (VCAM-1) is involved in the recruitment of leukocytes to inflammatory sites. In this study we present the first functional knockdown of VCAM-1 using an ER retained antibody construct. We generated a knockdown construct encoding the VCAM-1 specific single chain variable fragment scFv6C7.1 fused to the C-terminal ER retention sequence KDEL. HEK-293:VCAM-YFP cells stably expressing a VCAM-YFP fusion protein were transiently transfected with the knockdown construct and showed down-regulation of surface VCAM-1. Knockdown efficiency of the system is time-dependent due to used transient transfection of the intrabody construct. Furthermore, intrabody mediated knockdown of HEK-293:VCAM-YFP cells also impaired cell-cell interaction with Jurkat cells that are endogenously expressing VLA-4, the physiological partner of VCAM-1. Posttranslational knockdown with ER retained antibodies seems to be a promising technique, as shown here for VCAM-1.