Type III secretion system

A protein appendage found in several Gram-negative bacteria

The Type III secretion system (T3SS) is a complex protein appendage found in several Gram-negative bacteria. It is used by these bacteria to inject effector proteins directly into eukaryotic cells, which can manipulate host cell functions to the advantage of the bacteria. The T3SS is structurally similar to the flagellum, and it is believed that both systems evolved from a common ancestor.

Structure

The T3SS is composed of several components that form a needle-like structure. This structure is often referred to as the "needle complex" and is responsible for the translocation of proteins across the bacterial and host cell membranes.

Needle Complex

The needle complex is a multi-protein assembly that spans the bacterial inner and outer membranes. It consists of a basal body, a needle, and a translocon. The basal body anchors the needle complex to the bacterial membranes, while the needle protrudes from the bacterial surface and penetrates the host cell membrane.

Transmission electron micrograph of isolated T3SS needle complexes.

Basal Body

The basal body is a cylindrical structure that provides the foundation for the needle complex. It is composed of several rings that are embedded in the bacterial membranes. The basal body is similar in structure to the basal body of the bacterial flagellum.

Diagram of the basal body of a T3SS needle complex

Needle

The needle is a hollow tube that extends from the basal body. It is composed of polymerized proteins that form a rigid structure capable of penetrating host cell membranes.

Diagram of the T3SS needle complex.

Translocon

The translocon is a protein complex that forms a pore in the host cell membrane, allowing effector proteins to be delivered directly into the host cell cytoplasm.

Function

The primary function of the T3SS is to deliver effector proteins into host cells. These effector proteins can alter host cell processes, such as cytoskeleton dynamics, immune response, and cell signaling, to facilitate bacterial infection and survival.

Evolution

The T3SS is believed to have evolved from the bacterial flagellum, as both systems share structural similarities and components. This evolutionary relationship suggests that the T3SS and flagellum may have originated from a common ancestral secretion system.

Diagram of the base of a bacterial flagellum, showing structural similarities to the T3SS.

Related pages

• Flagellum
• Gram-negative bacteria
• Effector protein