Quantification of Shigella IcsA required for bacterial actin polymerization

Area of Science:

• Microbiology
• Cell Biology
• Infectious Diseases

Background:

• Shigella utilizes host actin polymerization to form actin tails for intracellular motility.
• The Shigella protein IcsA is known to mediate actin tail assembly.
• Previous studies using Xenopus egg extracts showed wild-type Shigella failed to assemble actin, a discrepancy not fully understood.

Purpose of the Study:

• To investigate the reasons behind wild-type Shigella's inability to assemble actin in Xenopus egg extracts.
• To determine the threshold levels of IcsA required for actin assembly by Shigella.
• To correlate IcsA expression levels in vitro and in vivo with actin assembly efficiency.

Main Methods:

• Utilizing IcsA-expressing Escherichia coli and wild-type Shigella in Xenopus cytoplasmic extracts.
• Manipulating IcsA expression levels to assess rescue of actin assembly defects.
• Quantifying IcsA molecules per bacterium required for actin filament and tail formation.

Main Results:

• Increasing IcsA expression by approximately 3-fold rescued the actin assembly defect in Xenopus extracts.
• Estimated 1,500-2,100 IcsA molecules per bacterium are needed for actin filament assembly, and ~4,000 for actin tail formation.
• Wild-type Shigella grown in vitro typically express lower IcsA levels than required, but expression increases ~3.2-fold in host cells.

Conclusions:

• The number of IcsA molecules per bacterium is critical for actin assembly.
• In vivo IcsA expression levels in Shigella-infected host cells are sufficient to promote actin tail formation.
• This study reconciles the discrepancy in Shigella actin assembly between in vitro extracts and infected host cells.