Transmembrane signaling is a crucial process that allows cells to sense and communicate with their external environment. One of its key components is autoproteolysis, where a peptide chain cleaves itself at a specific site. Until recently, this process was believed to be exclusive to eukaryotes, but researchers from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) of the Chinese Academy of Sciences (CAS) have discovered its presence in the bacterial species Clostridium thermocellum (C. thermocellum).
The researchers found that C. thermocellum utilizes autoproteolysis in transmembrane signaling, similar to eukaryotes. This groundbreaking discovery was published in Science Advances on July 7. Dr. Chen Chao, the first author of the study, noted that the conserved site for automatic cleavage suggests a more intricate world of bacterial signaling than previously thought.
Autoproteolysis in transmembrane signaling is rare in prokaryotes and is typically associated with protease maturation. However, in C. thermocellum, a bacterium that feeds on tough plant material (lignocellulolytic), it plays a vital role in signal transduction. This process occurs within the periplasm, the “in-between” spaces of the inner cytoplasmic membrane and bacterial outer membrane, in a conserved amino acid sequence known as asparagine-proline.
The conserved sequence is found on an anti-σ factor called “RsgI,” which is responsible for sensing and transducing signals to the cells. RsgI inhibits the activities of σ-factor “SigI,” which initiates RNA transcription from a DNA template and transcribes specific genes.
Lignocellulolytic bacteria like C. thermocellum possess extracellular enzyme complexes called “cellulosomes,” which are regulated by multiple pairs of RsgI/SigI factors. Cellulosomes help degrade tough plant materials, such as cellulose, hemicellulose, and pectin, that these bacteria consume.
The unique mechanism of autoproteolysis seen in C. thermocellum calls for further investigation into how it functions within lignocellulolytic bacteria, especially in the context of the cellulosome and the regulatory actions performed by anti-σ/σ factors, such as the binding of RNA polymerase to the promoter region to initiate transcription. Prof. Feng Yingang, the corresponding author of the study, emphasized that the functional mechanism of SigI/RsgI is yet to be fully elucidated.
Source: Chinese Academy of Sciences