New insights into the effects of membrane proteins on plant growth

Scientists have revealed two enzymes that regulate protein degradation in plant cell membranes and established the roles they play in plant growth and development.

Ubiquitin is a small regulatory protein found in most cells of eukaryotic organisms. It is vital for altering the function of other proteins and is notably involved in protein degradation and protein localization in the cell. Ubiquitin regulates these functions by attaching to and detaching from target proteina process called ubiquitination.

Scientists from Japan, Belgium and the United States, led by Associate Professor Takeo Sato of Hokkaido University, have discovered the first examples of deubiquitination enzymes in plants that act on cell membrane proteins of the cell. Their findings, published in the journal EMBO reportsdetail the function of these proteins.

The addition of ubiquitin to target proteins is catalyzed by enzymes called ubiquitin ligasesand the reverse process, removal of ubiquitin from target proteins, is catalyzed by deubiquitinating enzymes (DUB). It was unknown which DUB plants were able to directly remove ubiquitin from membrane proteins. This lack of knowledge meant that the regulation of membrane protein stability was not fully understood.

The team demonstrated that two of the Arabidopsis thaliana DUBs, UBP12 and UBP13, directly target the plant hormone receptor located in the cell membrane known as BRI1.

BRI1 is vital for the detection of brassinosteroids (BR), steroidal phytohormones essential for growth and development. Typically, when BRI1 detects BR, it triggers a pathway that regulates gene expression in the core. The abundance of cellular BRI1 is crucial for the precise mediation of the BR signal, although the regulating mechanism how BRI1 abundance is optimized (refined) in cells was unclear. Scientists demonstrated that UBP12 and UBP13 deubiquitinated and stabilized BRI1.

In their experiments, they showed that Arabidopsis thaliana plants that lacked the ability to express UBP12 and UBP13 were severely dwarfed and significantly less sensitive to BRs. When a BRI1 mutant – which could not be ubiquitinated – was introduced into these UBP12 and UBP13 deficient plants, the growth defects were partially restored. Specifically, UBP12 and UBP13 target and act on the ubiquitinated BRI1.

The study showed that UBP12 and UBP13 are key regulators of plant growth via their interaction with BRI1. The study also provided insight into how membrane protein stability is maintained; however, further research is needed to fully understand the dynamics of BRI1 within the cell. Finally, the results provide evidence that plant DUBs have similar roles to mammalian DUBs.