The Natural Nano-Engineering of Productivity

From factories to apps on cellphones, the concept of productivity seems everywhere. Is this concept a pure human invention?

The answer is complex and depends on one’s perspective:
From an industrial perspective, yes we can thank the flair of groups of engineers during the first industrial revolution, and especially the team of Henry Ford. However, in living organisms the optimization of the production of bio-molecules is much older than any human engineering. Indeed, all cells optimize processes in living organisms.

The (re)invention of assembly lines

Before jumping to nano-scale into cells, let’s have a look at engineered productivity and assembly lines found in factories.

Figure 1: An automatic assembly line in the car industry. Photo credit: Mixabest

The concept of assembly lines was initially developed for large-scale automobile production just over a century ago. It was a game-changer in industrial mass-production.

Let’s resume how an assembly line works in three points:

1) To complete a product in the shortest time frame, use sequential assembly lines.

2) Limit the motion of each component.

3) Optimize every move of specialized workers.

Nowadays, robots add elements at high precision and speed in fully automated assembly lines. But are those robots the edge of what is possible?

Nature Does Microscopic and Optimized

Living organisms, including your body, are made of cells. These cells are like microscopic factories that produce, transform, import, and export various products.

Discovering what happens in those tiny cellular factories is challenging. Indeed, cells are very good at keeping secrets. But biologists have made the art of extracting those secrets a scientific discipline, and developed tools to observe and track inputs and outputs of these microscopic living factories.

From all this tracking, we know that cells produce chemicals along biochemical pathways that are molecular assembly lines. These pathways need “workers” and employ enzymes, robot-like proteins that carry out step by step assembly towards completion of the desired chemical structures. These enzymes are so small (1–100 nanometer with 1nm=1 millionth of a mm) that they classify as nano-particles.

A biochemical pathway present in Arabidopsis plants. The product, Camalexin, is an antifungal defense compounds. The names of the enzymes are labeled in red, with arrows representing production steps. Adapted from Figure 1 from Mucha et al. 2019, The Plant Cell.

The Metabolon: The Mass-Production Assembly Line

Some biological pathways need to be extremely fast and reliable. One example is pathways that produce chemical defense for which fast product release = survival. Such fast-paced pathways constitute “metabolic highways”. Recent studies suggest that some of these fast-paced pathways are based on protein complexes called metabolons.

As in engineered assembly lines, in metabolons the transport of components is reduced to near zero as enzymes work in close contact. Enzymes also adapt their workload to create a highly coordinated team in comparison to single workers. They can also recruit additional specialist enzymes when needed.

Doesn’t that sound fascinating?

Let’s zoom into a plant cell and figure out how one such nano-structure works. Mucha and coauthors recently published in The Plant Cell the structure of a plant defense metabolon [1]. The researchers studied the production of camalexin, a toxic alkaloid. This compound is a key defensive weapon of Arabidopsis thaliana, a well-studied plant.

A hypothetical schematic of the camalexin metabolon. The enzymes of this protein complex are anchored into a membrane (brown 3D structure below). Reproduced from Mucha et al. 2019 Figure 10.

Why does it matter to the plant to produce camalexin efficiently?

When working as a team, enzymes gain efficiency. This is important as camalexin is produced out of tryptophan, an essential amino acid. The use of tryptophan is a massive investment of key resources. It is such an investment that camalexin is produced only when absolutely needed and only around the site of infection. Under an optimal strategy plan, this investment pays off since camalexin slows down the progression of pests on leaves, acting as a chemical shield. But to be effective, camalexin has to reach massive amounts in a matter of hours. The camalexin metabolon makes this possible.

Fluorescence microscopy of a leaf under attack by Botrytis cinerea, a fungus. Camalexin is mass-produced in cells surrounding the infection site (In white). In green, the fluorescence indicates the production of camalexin. The yellow stars show where the infection started. The white bar shows 50um (0.05 mm or 0.002 inch). Reproduced from Figure 2C&F from Mucha et al. 2019, The Plant Cell.

The enhanced efficiency and yield gain are not the only benefits of the metabolon. The production of effective chemical weapons can involve the synthesis of intermediate forms that are even more toxic than the final product. If released, those compounds could kill the cell itself. Within the metabolon, the chemicals are modified without the release of such intermediates, avoiding self-poisoning.

It is fascinating to realize that human engineering only reinvented assembly lines. Nano-scale assembly lines have been present in nature for millennia. However, they were so well hidden in cells behind walls that humans had no clue.

Céline Caseys

Department of Plant Sciences

University of California, Davis

celcaseys@ucdavis.edu

Orcid: 0000–0003–4187–9018

Twitter: C_Caseys

References:

Mucha S, Heinzlmeir S, Kriechbaumer V, Strickland B, Kirchhelle C, Choudhary M, Kowalski N, Eichmann R, Huckelhoven R, Grill E, Kuster B, Glawischnig E. (2019). The formation of a camalexin-biosynthetic metabolon. The Plant Cell Doi: https://doi.org/10.1105/tpc.19.403