# Legionella effector protein SidG disrupts host cytoskeleton via targeting Arp2/3 complex

## TL;DR

The Legionella protein SidG helps the bacteria invade host cells by disrupting the cell's structural framework using a host protein called Rac1.

## Contribution

SidG's novel mechanism of using a Cys-His-Asp triad and Rac1 to target the Arp2/3 complex for cytoskeletal disruption is newly identified.

## Key findings

- SidG requires activation by the host GTPase Rac1 to function.
- SidG disrupts the Arp2/3 complex, which is essential for actin nucleation.
- SidG's Cys-His-Asp motif is critical for bacterial invasion of host cells.

## Abstract

Legionella pneumophila is a facultative intracellular bacterial pathogen capable of surviving and replicating within host cells, including macrophages and protozoans. It employs the Dot/Icm type IV secretion system (T4SS) to inject over 330 effector proteins into host cells, manipulating various cellular processes to facilitate infection. Characterizing the functions of these effectors is crucial to deciphering the pathogenesis of L. pneumophila. In this study, we identified SidG as an effector containing a Cys-His-Asp triad, whose functional state is strictly gated by its interaction with the cellular GTPase Rac1, particularly via its C-terminal domain. Rac1-activated SidG then utilizes an acidic (A) domain to target the Arp2/3 complex, the key regulator of actin nucleation. Importantly, SidG disrupts cytoskeletal architecture via both Rac1- and Arp2/3-dependent mechanisms. During L. pneumophila infection, SidG is crucial to promote efficient bacterial invasion of host cells in a Cys-His-Asp motif-dependent manner. Together, our study elucidates a sophisticated pathogenic mechanism where a bacterial effector co-opts a host GTPase to allosterically regulate its function towards the Arp2/3 complex, thereby facilitating bacterial entry into host cells.

To invade and proliferate within human immune cells, Legionella employs a sophisticated infection strategy to inject over 330 bacterial effector proteins directly into host cells to manipulate cellular processes. Among these effectors, we found that SidG plays a critical role in disrupting the host cytoskeleton, a network of protein filaments essential for maintaining cell structure and motility. Our results revealed that SidG functions as a tightly regulated molecular machine that remains dormant until triggered. Specifically, SidG initially binds to the host GTPase Rac1, which acts as a molecular switch to release SidG from its inactive state. Upon activation, SidG then targets the Arp2/3 complex and disrupts actin network. This dual regulatory mechanism allows Legionella to manipulate host cell architecture and facilitate bacterial entry into cells. Our findings uncover a sophisticated strategy in which pathogens hijack host cellular machinery to establish infection, providing new insights into Legionella pathogenesis and potential therapeutic targets.