Molecule of the Month: Injectisome

We live in symbiosis with a vast community of bacteria. This interaction is most often friendly. Bacteria in the environment degrade wastes and dead plants and animals. Bacteria in our bodies help us process our food and provide essential nutrients. Sometimes, however, the interaction is less amenable. For example, Salmonella bacteria invade our cells and reproduce inside, causing life-threatening diseases like typhoid fever. As part of their pathogenic strategy, these bacteria inject several dozen types of effector proteins using a molecular needle called the injectisome. These injected proteins disable the cell’s defenses and rewire the cell’s metabolism to assist with the growth of the bacteria inside the cell.

PDB entry 7ah9 includes the central needle portion of the Salmonella injectisome. It includes a thin needle with a central pore just large enough to deliver an unfolded protein chain. A molecular gate, shown in more detail in the JSMol image below, controls passage of effector proteins through the needle. The structure also includes several large rings of proteins that anchor the needle in the bacterial cell wall, allowing the bacterium to position several of them against a cell that it’s trying to infect.

The full injectisome also includes a large sorting platform inside the bacterium, which is not included in this structure. This platform selects only the appropriate effector proteins and manages the order in which they are injected. They are secreted in three stages. First, the components of the injectosome itself are secreted, building the needle and other components. Then a collection of proteins are secreted that create a translocon through the infected cell’s membrane. Finally, the injectosome docks with this translocon and injects the remaining effector proteins into the infected cell’s cytoplasm.

This structure was determined by cryoelectron microscopy and includes the effector protein SptP, an enzyme that removes phosphates from tyrosines on cellular proteins and also inactivates several proteins that are controlled by GTP. Researchers had to play some experimental games to get a view of the protein in transit, since effector proteins typically take less than a second to pass through the needle. To do this, they engineered SptP with green fluorescent protein attached at the end. The injectosome is unable to unfold GFP, so it forms an anchor that can’t fit through the needle, trapping the complex so it can be studied at leisure.

Dozens of effector proteins are injected by Salmonella bacteria. Many of them interrupt the action of cellular proteins by adding or removing phosphates, ribosyl groups, and other chemical modifications. Other effectors deliver key cellular proteins to the ubiquitination system for disposal. The effector protein shown here, GtgA, is a protease that cuts a specific transcription factor, p65 (PDB entry 6ggr). This transcription factor is part of the NF-κB signaling pathway that controls many processes related to inflammation and the immune system. The NF-κB complex shown here is a heterodimer of p65 and p50 (PDB entry 1le5) bound to DNA.