Structural basis for synthase activation and cellulose modification in the E. coli Type II Bcs secretion system.

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Bacterial cellulose (BC) is a crucial biofilm matrix exopolysaccharide.
• BC synthesis relies on complex bacterial cellulose secretion (Bcs) systems.
• These systems are regulated by cyclic diguanylate (c-di-GMP) and involve diverse protein subunits.

Purpose of the Study:

• To elucidate the molecular mechanisms of Bcs system assembly and regulation in E. coli.
• To understand the recruitment and stabilization of key Bcs subunits involved in cellulose modification and secretion.
• To investigate the role of BcsE as a secondary c-di-GMP sensor in regulating Bcs activity.

Main Methods:

• Cryogenic electron microscopy (cryo-EM) was employed to determine high-resolution structures.
• Structural analysis focused on the BcsA-dependent recruitment of BcsG and BcsF-dependent recruitment of the BcsE2R2Q2 complex.
• Biochemical assays were used to assess the binding of BcsE to c-di-GMP and its interaction with BcsRQ.

Main Results:

• Cryo-EM revealed the BcsA-dependent recruitment of a trimeric BcsG pEtN-transferase and BcsF-dependent recruitment of a dimeric BcsE2R2Q2 regulatory complex.
• BcsE, a secondary c-di-GMP sensor, can bind dinucleotides and stabilize essential BcsRQ partners on the synthase independently of direct c-di-GMP-synthase complexation.
• This stabilization mechanism lowers the activation threshold for the Bcs synthase, enabling activity even with low intracellular c-di-GMP.

Conclusions:

• The study provides atomic-level insights into the structural organization and regulation of the E. coli Bcs system.
• An 'activation-by-proxy' mechanism involving BcsE allows for efficient cellulose secretion under varying c-di-GMP conditions.
• This regulatory strategy is conserved in other polysaccharide secretion systems, highlighting a common principle in microbial physiology.