Sensing and communicating with the extracellular environment are both aspects of transmembrane signaling. An important mechanism known as autoproteolysis, which is essential to many cellular processes, is a component of transmembrane signaling.
In this procedure, a peptide, which is a chain of amino acids, cleaves the chain at a certain point. Although a distinct, functioning autoproteolytic process for transmembrane signal transduction has not yet been observed in bacteria, this is typical of eukaryotes.
Now, however, this signaling process thought to be almost entirely exclusive to eukaryotes has been confirmed in bacterial species Clostridium thermocellum (C. thermocellum), according to researchers from the Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) of the Chinese Academy of Sciences (CAS).
Researchers discovered an autoproteolytic effect in the bacteria C. thermocellum that is crucial for triggering later transmembrane signaling, comparable to the well-known autoproteolysis process seen in eukaryotes. The study was published in Science Advances on July 7, 2023.
As a unique class of σ/anti-σ factors, the functional mechanism of SigI/RsgI has still not been fully elucidated.
Professor Feng Yingang
“Not only is this a novel observation, but the presence of a conserved site for the automatic cleavage of the amino acid chain suggests a more unique and complex world of bacterial signaling than many would’ve thought possible,” said Dr. Chen Chao, first author of the study.
Prokaryotes rarely report autoproteolysis for transmembrane signaling, and it is typically reserved for protease maturation. But in the case of C. thermocellum, a bacterium that feeds off of tough plant material (lignocellulolytic), it appears to be an essential part of signal transduction.
Asparagine-proline, a conserved amino acid sequence, is used in the process, which takes place between the “in-between” regions of the inner cytoplasmic membrane and bacterial outer membrane known as the periplasm.
The anti-σ factor “RsgI,” a protein that senses and transmits signals to cells, contains this conserved sequence. It also works to inhibit the actions of the anti-σ factor “SigI,” which initiates the transcription of RNA from a DNA template and is responsible for transcribed specific genes.
Lignocellulolytic bacteria, like the one used in this study, C. thermocellum, have extracellular enzyme complexes called “cellulosomes,” which are the targets regulated by numerous pairs of RsgI/SigI factors. The cellulosome functions to break down the hard plant materials the bacteria consume, like cellulose, hemicellulose, and pectin.
“As a unique class of σ/anti-σ factors, the functional mechanism of SigI/RsgI has still not been fully elucidated,” said Prof. Feng Yingang, corresponding author of the study.
It is reasonable to assume that further research is needed to learn more about the specifics of how autoproteolysis in lignocellulolytic bacteria functions, particularly with regard to the cellulosome and the accompanying regulatory actions carried out by anti-/σ/σ factors, such as the binding of RNA polymerase to the promoter region to start transcription. This is because C. thermocellum exhibits a unique mechanism for autoproteolysis.
