The Venus flytrap’s major protease

Published May 4 2016

Venus flytraps grow in the nitrogen-poor soil of the southeastern United States. The lack of nutrition in the soil has turned these plants into sophisticated hunters. Two lobes of the flytrap produce sweet sap that lures small insects. If an insect brushes against a few microscopic, hair-like structures on the lobes’ surface in quick succession, the lobes spring shut instantaneously to trap the insect. Then the plant secretes a fluid that kills and digests the prey.

Carnivorous Venus flytrap plants secrete enzyme-rich fluid to mediate nutrient absorption from their prey.WIKIMEDIA COMMONS USER NOAH ELHARDT

The flytrap’s digestive fluid is a rich concoction of enzymes including lipases, chitinases and proteases. In a recent paper in the Journal of Biological Chemistry, Jan Enghild and a team of researchers at the department of molecular biology and genetics at Aarhus University in Denmark, characterized the function and structure of a 45 kDa cysteine protease, dionain-1, which is found abundantly in the digestive fluid. The protein has a precursor form called pro-dionain-1. Pro-dionain-1 contains an N-linked glycan group, which is required for proper folding of the protein. The researchers used cDNA sequence analysis to demonstrate that pro-dionain-1 is similar to the precursors of other plant cysteine proteases, such as papain.

Further analysis revealed that pro-dionain-1 is also homologous to propapain at the structural level. Despite the sequence and structural similarity, pro-dionain-1’s function is better suited to acidic environments than papain. Pro-dionain-1 undergoes autoproteolysis at an acidic pH. The autoproteolysis process leads to loss of the integrity of the prodomain and unravels the active site to produce the functional and mature dionain-1. The authors highlight that, unlike other proteases that require acidic pH only for maturation, dionain-1 must exert its main proteolytic function at the acidic pH of 3.4 to 4.4 found in the digestive sap of Venus flytraps.

The mature dionain-1 efficiently digests lysine- and arginine-rich muscle proteins to release nitrogen-rich peptides. Thus, the authors conclude that dionain’s function is finely tuned to serve the plant’s need for nitrogen-rich sources. According to the study’s lead author, Michael Risør, dionain’s enhanced acid tolerance and its preferential digestion of nitrogen-rich proteins is an important evolutionary adaption that facilitated the flytrap’s carnivorous lifestyle.

Enghild, who oversaw the work, says that the research on the digestive fluid offers insight not only into the mechanisms of plant carnivory but also how those mechanisms differ from molecular strategies employed by carnivorous animals. In the future, Enghild and his team plan to investigate other enzymes in the flytrap’s digestive fluid.

Indumathi Sridharan Indumathi Sridharan earned her bachelor’s degree in bioinformatics in India. She holds a Ph.D. in molecular biochemistry from Illinois Institute of Technology, Chicago. She did her postdoctoral work in bionanotechnology at Northwestern University.