Question

Compare and contrast Type II, Type III and Type IV secretion systems. In particular, describe similarities and differences in their structure, evolution and mechanism of secretion. (Microbiology)

Answer 1

Secretion systems are protein complexes on cell membranes of bacteria for secretion of substances.

Type II System: Also called as T2SS system.

Secretion apparatus consists of Secretin and Inner membrane protein. There are 4 components, viz,

Outer membrane, Inner membrane, Secretion of ATPase and Pseudo Pili.

It is a two step process

Exoproteins are secreted from Cytoplasm to periplasm using sec and tat system

Secretion out of the cell through secretary protein.

Exoproteins are secreted out of cytoplasm to periplasm. Pseudopilins are cleaved using Prepilin peptidase GspO into matured pseudopilin. This can insert themselves into inner membrane. ATPase GspE hydrolyses ATP, releases energy and form pseudopilus assembly. This forces the exoprotein out through the secretin GspD. Secretin forms hydrophilic channel in the outer membrane.

Type III system: Also called TTSS or T3SS

Structurally similar to basal body of flagella consists of three components namely Needle, Inner rode and base. Found in Gram negative bacteria such as Salmonella. This is discovered from Yersinia pestis. Bacteria injects toxins directly from the bacterial cytoplasm to cytoplasm of the host cell. T3SS protein complex include Structural proteins, effector protein and chaperons.

Effector is identified by the initial aminoacid sequences by the structural protein, Contact of needle with the host cell triggers secretion.

Type IV system: Also called T4SS system.

This is related to bacterial conjugation system. It can transport single proteins or Protein-protein complex or DNA-protein complex. Originally discovered in Agrobacterium tumifacians. Here it uses the system to transfer T DNA portion of the Ti plasmid into plant cells in which a crown gall is produced. It is a major fitness determinent of interspecies bacterial competition. It helps in bacterial conjugation, DNA exchange with extracellular space and delivery of protein into target cell.

Structurally there are 12 protein units in this system viz. Vir B1 to Vir B11 and Vir D. Functions of important proteins are as below.

Vir B6 to Vir B10 forms the translocation channel, the channel between extracellular space and cytoplasm.

Vir B4, Vir B11 and Vir D forms ATPase, which drives substrate motion through the translocation channel.

Vir B2 and Vir B5 forms the Pilus.

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