Decoding the Great Expansin Mystery: How Plant Proteins Wound Up in Bacteria
"Unraveling the evolutionary journey of expansins in Streptomyces and Kutzneria – a tale of gene transfer, adaptation, and potential biotech innovation."
Expansins, a protein superfamily predominantly found in plants, also appear in bacteria, fungi, and amoebozoa. In plants, these proteins play a crucial role in cell wall loosening by binding to the cell wall and relaxing cellulose microfibrils without enzymatic action. This mechanism is essential for plant growth and development.
The presence of expansins outside the plant kingdom has long puzzled scientists. The evolutionary history of these non-plant expansins is complicated by horizontal gene transfer (HGT), a process where genetic material is transferred between organisms that are not parent and offspring. This makes it difficult to pinpoint the precise origin and function of expansins in bacteria and other organisms.
Recent research has uncovered a fascinating case of plant-like expansins in two bacterial genera: Streptomyces and Kutzneria. This discovery provides a unique opportunity to delve deeper into the evolution of expansins and their potential roles in bacterial physiology and interactions with other organisms.
How HGT Is Studied
Horizontal gene transfer (HGT) is studied through experimental and theoretical approaches that track the movement of genetic information within and between microbial populations. Unlike vertical transmission from parent to offspring, HGT exchanges genetic material laterally across species boundaries. The field also examines limits on transfer: a study led by Christina Burch and Corbin Jones at the University of North Carolina investigated evidence that genetic complexity can restrict HGT. These approaches therefore treat HGT as both a driver of microbial evolution and a process whose success varies with biological constraints.
The Curious Case of Bacterial Expansins
Researchers conducted a comprehensive genome-wide search for HGT events in Streptomyces species. This search led to the identification of a plant-like expansin within the proteome of Streptomyces acidiscabies, a bacterium known for causing potato scab. Further investigation revealed the presence of a similar protein in Kutzneria sp. 744.
- Structural Similarity: Despite their bacterial hosts, the 3D structures of the bacterial expansins show similarities to plant expansins, particularly in their electrostatic potentials and folding patterns.
- Distinct from Canonical Expansins: These expansins differ from both plant and bacterial canonical expansins, suggesting they've undergone specific adaptations since their transfer.
- Plant Origin Confirmed: Analysis points towards a plant origin for these bacterial expansins, setting them apart from typical bacterial expansins.
A Broad and Ancient Protein Family
Recent reviews describe expansins as a superfamily found mainly in plants but also in bacteria, fungi, and amoeba. Their presence in cellulose-utilising eukaryotic microorganisms has been interpreted as evidence of an evolutionary origin in ancient marine microbes predating land plants. Research on HGT likewise emphasizes that genetic exchange can accelerate evolutionary rates, facilitate adaptive innovations, and shape microbial pangenomes. Together, these findings place bacterial expansins within a broader history of protein distribution across widely separated lineages.
Why the Evolutionary Story Is Not Simple
The distribution of bacterial expansin-like proteins is uneven: BPLEAs were observed in nine strains of Streptomyces acidiscabies but only in strain 744 of Kutzneria sp. 744. A microbial expansin phylogeny has been reported to show multiple HGT events within and between bacterial and eukaryotic microbial lineages, which could help explain the family's broad phylogenetic distribution. However, comparative studies indicate that individual genes differ in how successfully they undergo HGT, and genetic complexity can limit transfer. The strain-level pattern therefore does not by itself establish a single, universal route for bacterial expansin evolution.
Why This Matters and What's Next
The discovery of plant-like expansins in bacteria raises several intriguing questions about their function and evolution. What role do these expansins play in the biology of S. acidiscabies and Kutzneria sp. 744? How did they adapt to function in a bacterial context?
Transfer Depends on Context
HGT is the non-sexual movement of genetic information between genomes and has played a pronounced role in bacterial and archaeal evolution. Quantitative analyses report that transfer rates determine how efficiently and rapidly traits spread through bacterial populations, with rates depending strongly on the transfer mechanism and environment. Work on plant expansins has also used protein BLASTp searches against the NCBI nonredundant database to test whether EXLX1 homologs occur in nonplant species. These approaches support a context-dependent interpretation in which both transfer dynamics and sequence evidence matter when evaluating plant-like proteins in bacteria.
HGT Across Human Microbiomes
A study of thousands of bacterial strains from 15 human populations examined how HGT rates and targets vary across hosts with different levels of industrialization. It reported that HGT had accumulated in the microbiome over recent host generations. The study also found that HGT occurs at high frequency within individuals. These results show that gene exchange is shaped by population and environmental context, not only by the properties of an individual gene.
Further research is needed to fully understand the functional significance of these bacterial expansins. Experimental studies could reveal their specific roles in bacterial cell wall modification, biofilm formation, or interactions with plant hosts.
Moreover, these unique bacterial expansins could have potential applications in biotechnology. Their ability to modify cell walls could be harnessed for various purposes, such as improving biomass processing or developing novel drug delivery systems. As more plant and bacterial genomes become available, scientists will be able to validate, improve, or change the proposed hypotheses, the uniqueness of BEAPLs turns these proteins interesting for further experimental research and applications in biotechnology.