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Detecting Cortex Fragments During Bacterial Spore Germination
Published on: June 25, 2016
The CspC:CspA heterodimer transduces germinant and co-germinant signals during Clostridioides difficile spore
Morgan E McNellis1,2, Gonzalo González-Del Pino1, Juan A Serrano-Jiménez1
1Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, Massachusetts, United States of America.
Abstract:
The clinically significant pathogen Clostridioides difficile lacks the transmembrane nutrient germinant receptors conserved in almost all spore-forming bacteria. Instead, C. difficile initiates spore germination using a unique mechanism that requires two signals: a bile acid germinant and a co-germinant, which can be either an amino acid or a divalent cation. While two soluble pseudoproteases, CspC and CspA, were initially identified as the germinant and co-germinant receptors, respectively, in C. difficile, we previously identified residues in an unstructured region of CspC that regulate the sensitivity of C. difficile spores to both signals. However, the mechanism by which CspC transduces these signals remained unclear. Here, we demonstrate that CspC forms a stable complex with CspA and determine the crystal structure of the CspC:CspA heterodimer. The structure reveals extensive interactions along the binding interface, including direct interactions between the unstructured region of CspC and CspA. Using structure-function analyses, we identify CspC:CspA interactions that regulate the sensitivity of C. difficile spores to germinant signals and show that CspA regulates the response of C. difficile to not only co-germinant but also germinant signals. While we show that CspA can form a homodimer and determine its crystal structure, CspA homodimerization appears unimportant for C. difficile spore germination. Collectively, our analyses establish the CspC:CspA heterodimer, rather than its individual constituents, as a critical signaling node for sensing both germinant and co-germinant signals. They also suggest a new mechanistic model for how C. difficile transduces germinant signals, which could guide the development of therapeutics against this important pathogen.
Insights
Clostridioides difficile spore germination relies on a unique CspC:CspA heterodimer complex. This complex acts as a signaling hub, sensing both germinant and co-germinant signals to control spore activation.
Area of Science:
- Microbiology
- Structural Biology
- Molecular Mechanisms
Background:
- Clostridioides difficile spores utilize a unique germination pathway involving bile acid germinants and co-germinants (amino acids or divalent cations).
- Previous studies identified CspC and CspA as key pseudoproteases involved in sensing these signals, but the precise mechanism of signal transduction remained elusive.
- The CspC protein contains an unstructured region crucial for regulating spore sensitivity to germination signals.
Purpose of the Study:
- To elucidate the structural basis and mechanism of signal transduction by CspC and CspA in Clostridioides difficile spore germination.
- To determine the role of the CspC:CspA complex in sensing both germinant and co-germinant signals.
- To propose a novel mechanistic model for Clostridioides difficile spore germination.
Main Methods:
- Crystallography to determine the structure of the CspC:CspA heterodimer and CspA homodimer.
- Structure-function analyses to investigate the role of specific interactions within the CspC:CspA complex.
- Biochemical assays to assess spore germination sensitivity to various signals.
Main Results:
- The crystal structure of the CspC:CspA heterodimer revealed extensive interactions at the binding interface, including direct contacts involving CspC's unstructured region.
- Specific CspC:CspA interactions were identified as critical regulators of spore sensitivity to germinant and co-germinant signals.
- CspA was shown to regulate the response to both germinant and co-germinant signals, and CspA homodimerization was found to be non-essential for germination.
Conclusions:
- The CspC:CspA heterodimer functions as a crucial signaling node for sensing both germinant and co-germinant signals in Clostridioides difficile.
- A new mechanistic model for signal transduction in Clostridioides difficile spore germination has been proposed.
- Understanding this unique mechanism could lead to the development of novel therapeutics against Clostridioides difficile infections.
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