When intracellular, pathogenic Salmonella reside in a membrane compartment composed of interconnected vacuoles and tubules, the formation of which depends on the translocation of bacterial effectors into the host cell. Cytoskeletons and their molecular motors are prime targets for these effectors. In this study, we show that the microtubule molecular motor KIF1Bβ (a splice variant of KIF1B), a member of the kinesin-3 family, is a key element for the establishment of the Salmonella replication niche as its absence is detrimental to the stability of bacterial vacuoles and the formation of associated tubules. Kinesin-3 interacts with the Salmonella effector SifA but also with SKIP (also known as PLEKHM2), a host protein complexed to SifA. The interaction with SifA is essential for the recruitment of kinesin-3 on Salmonella vacuoles whereas that with SKIP is incidental. In the non-infectious context, however, the interaction with SKIP is essential for the recruitment and activity of kinesin-3 only on a fraction of the lysosomes. Finally, our results show that, in infected cells, the presence of SifA establishes a kinesin-1 and kinesin-3 recruitment pathway that is analogous to and functions independently of that mediated by the Arl8a and Arl8b GTPases. This article has an associated First Person interview with the first author of the paper.

Cells infected with Salmonella are characterised by the appearance of membrane tubular structures that stretch from the bacterial vacuole. The formation of these tubules requires the translocation of Salmonella effector proteins within the infected cell. Different types of Salmonella-induced tubules with varying host protein compositions have been identified. This variability probably reflects the ability of these tubules to interact with different host compartments. Membrane tubules decorated with effector proteins but essentially devoid of host proteins and named LAMP1-negative (LNT) were observed. LNTs wrap around LAMP1-positive vesicles and may promote recruitment of lysosomal glycoproteins to bacterial vacuole and the formation of a replication niche. We conducted a biochemical and functional characterisation of LNTs. We show that the effector proteins SseF and SseG are necessary for their formation. The absence of these tubules is associated with decreased recruitment of LAMP1 to SCVs, decreased intracellular replication of Salmonella, and decreased virulence in mice. We found that the process leading to the recruitment of lysosomal glycoproteins to tubules involves the C-terminal domain of the effector protein SifA and the GTPase Arl8b. Overall, these data suggest that Salmonella-induced tubules promote the establishment of the replication niche by promoting recruitment of host proteins to the bacterial vacuole.

The virulence of Salmonella relies on the expression of effector proteins that the bacterium injects inside infected cells. Salmonella enters eukaryotic cells and resides in a vacuolar compartment on which a number of effector proteins such as SifA are found. SifA plays an essential role in Salmonella virulence. It is made of two distinct domains. The N-terminal domain of SifA interacts with the host protein SKIP. This interaction regulates vacuolar membrane dynamics. The C-terminal has a fold similar to other bacterial effector domains having a guanine nucleotide exchange factor activity. Although SifA interacts with RhoA, it does not stimulate the dissociation of GDP and the activation of this GTPase. Hence it remains unknown whether the C-terminal domain contributes to the function of SifA in virulence. We used a model of SKIP knockout mice to show that this protein mediates the host susceptibility to salmonellosis and to establish that SifA also contributes to Salmonella virulence independently of its interaction with SKIP. We establish that the C-terminal domain of SifA mediates this SKIP-independent contribution. Moreover, we show that the two domains of SifA are functionally linked and participate to the same signalling cascade that supports Salmonella virulence.

Bacteria of the genus Salmonella express nanosyringe-like organelles called type three secretion systems (T3SSs). These systems promote the secretion of bacterial compounds and their translocation into host cells. Pathogenic Salmonella use two distinct T3SSs, with specialized functions, having the purpose to modify the biology of the host organism and to ensure a successful infection. The bacterial proteins translocated through the first T3SS (T3SS-1) facilitate the entry of Salmonella into host cells, whereas T3SS-2 is an important factor for shaping the intracellular replication niche. In addition both T3SSs have a strong impact on the host inflammation. For a long time the two T3SSs were thought to act separately. However, there is increasing evidence that their regulation depends not only on separate but also shared regulatory mechanisms and that their time of action during infection overlaps. Here, we review the current understanding of the structure and of the regulation of expression and activity of both T3SSs. The output image is multifaceted, as recent studies show that subpopulations of Salmonella present diverging patterns of expression and activity of T3SSs during important steps of infection. These diversities may advance the chances of Salmonella to outpace competitors and to well establish itself in its niche in the host.