Molecular Caps Keep Internal Cell Structures Intact—Or Not

Biological Strategy

Eukaryotes

Eleanor Banwell

Image: Tom Deerinck / Flickr / Some rights reserved

Modify Position

Many resources that living systems require for survival and reproduction constantly change in quantity, quality, and location. The same is true of the threats that face living systems. As a result, living systems have strategies to maintain access to shifting resources and to avoid changing threats by adjusting their location or orientation. Some living systems modify their position by moving from one location to another. For those that can’t change location, such as trees, they modify position by shifting in place. An example of an organism that does both is the chameleon. This creature can move from place to place to find food or escape predators. But it also can stay in one place and rotate its eyes to provide a 360-degree view so that it can hunt without frightening its prey.

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Modify Size/Shape/Mass/Volume

Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.

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Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Regulate Reproduction or Growth

Reproduction and growth are two physiological processes that occur in all living systems. There are situations when conditions are right for both, and other situations when continuing either harms the living system because both have a very high energy cost. Reproduction and growth are unique in that both can stop until conditions improve, although stopping either for an extended time can cause problems. An example of regulating reproduction is a process called delayed implantation or embryonic diapause found in some mammals, such as otters. An otter’s embryos sometimes temporarily cease developing and won’t develop further until the female senses that conditions are suitable.

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Regulate Cellular Processes

Cells are the basic building blocks of all living systems, so cellular processes dictate how physiological processes occur within those systems. Cells (whether entire unicellular organisms or parts of multicellular living systems) grow, metabolize nutrients (that is, chemically transform them), produce proteins and enzymes, replicate, and move. Cells as part of multicellular systems rarely act alone, instead having ways to signal to start and complete simple to quite complex interactions. How skin heals is a good example of the role of cellular processes. Blood cells called platelets release clotting factors to stop the bleeding; white blood cells rid the area of foreign materials and release molecules to coordinate healing; cells called fibroblasts start rebuilding using proteins called collagen; new blood vessels form; and skin cells called keratinocytes create the new surface.

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Animals

Kingdom Animalia (“having breath or soul”): Mammals, reptiles, sponges, corals, insects, birds

This kingdom captures a staggering variety of life—from the smallest coral polyp to the largest elephant. Although animal body shapes and life histories vary, there are a few defining characteristics. Animals are heterotrophs, meaning they gain energy from eating other organisms, and have developed from single-celled ancestors to highly diversified and coordinated multicellular forms. Unlike plant and fungi cells that have rigid cell walls, animal cells are bound by more flexible combinations of proteins. Most animals have cells organized into tissues and are able to move within their environment.

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Microtubules fray when protective molecules degrade, then grow again when they reform.

Eukaryotic cells are packed full of microtubules. Far from being a loose sac of water and chemicals, as they are often envisaged, cells are actually densely packed with structures and organelles, all constantly carrying out crucial functions. While a lot of these functions are regulated by the passive diffusion of signaling molecules, many of them require active transport. Microtubules make up the physical skeleton of cells, enabling them to move and change shape, and they also form part of the infrastructure along which cellular components are transported. Microtubules also transport chromosomes, and so are critical for cell division and replication.

Microtubules are formed from numerous copies of the tubulin. Loose tubulin in the cytoplasm binds to molecules of GTP and becomes activated. In its active form, individual tubulin proteins will attach to one end of a growing microtubule. However, tubulin breaks down (hydrolyzes) GTP to GDP and GDP-bound tubulin is much more likely to dissociate from a microtubule. This reaction does not occur immediately, and there is a delay between tubulin becoming bound to GTP and its hydrolyzation to GDP. Because GDP-bound tubulin cannot pop out of the side of a microtubule in which it is already incorporated and only dissociates from one end, as long as new tubulin is being added to the end of a microtubule at a faster rate than hydrolyzation is occurring, there will be a cap of GTP-bound tubulin that will prevent the microtubule falling apart. However, if the rate of new tubulin addition slows and the protein at the tip hydrolyzes its GTP to GDP, this protective cap is lost and the microtubule will begin to fray. This sudden switch from growing to shrinking is called a microtubule “catastrophe” and is an important part of regulating tube length. Once a cap of GTP-bound tubulin reforms, the microtubule begins to grow again, termed “rescue”.

The precise mechanisms regulating microtubule catastrophe are not yet known, although it is understood to be regulated by multiple different factors. Catastrophe occurs at a higher rate in longer microtubules, indicating there must be a multi-step process that protects shorter microtubules from undergoing frequent catastrophe and enables them to grow.

To see how microtubule catastrophe works, check out this video.

Heavily Loaded Transport Protein Catch-bonds

Common to all organisms

Transport protein in eukaryotes binds more tightly to its track via molecular force transmission only when heavily loaded

Image: Thomas Splettstoesser / CC BY SA - Creative Commons Attribution + ShareAlike

Last Updated April 20, 2018

References

“Microtubules are essential players in the function of the cell. Together with actin filaments and intermediate filaments, they comprise the cytoskeleton in eukaryotic cells, and this group of polymers is collectively responsible for providing most of the structure and spatial organization in the cell. Microtubules are also involved in transport, migration and reorganization and have numerous dynamic roles, including movement via motor proteins such as kinesin and dynein, the beating of cilia and flagella, and the segregation and separation of chromosomes during cell division” (Sept 2007:R764)

Journal article

Microtubule Polymerization: One Step at a Time

Current Biology, 17(17): R764-R766 | 04/09/2007 | Sept D

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Reference

Journal article

Microtubule Assembly Dynamics at the Nanoscale

Current Biology, 17(17): 1445-1455 | 04/09/2007 | Schek HT III; Gardner MK; Cheng J; Odde DJ; Hunt AJ

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Reference

“The dynamic behavior at the end of an individual microtubule is termed ‘‘dynamic instability’’. This behavior manifests itself by periods of persistent microtubule growth interrupted by occasional switching to rapid shrinkage (called microtubule ‘catastrophe’), and then by switching back from shrinkage to growth (called microtubule ‘rescue’)” (Gardner et al. 2013:14)

“A microtubule ‘catastrophe’ event manifests itself by the sudden switch of a growing microtubule into a rapidly shortening state. The widely accepted view of microtubule catastrophe is that it involves a single random event, such as the sudden loss of a protective end structure [1–3]. This single-step mechanism implies that a microtubule has the same probability of undergoing catastrophe at any given point in time, irrespective of how long it has been growing already…However, measurements of microtubule length and lifetime distributions both in vitro and in vivo do not display a simple exponential decay [4–13]. Although the distribution of microtubule lengths appears to decay exponentially at longer lengths, the predicted exponential distribution is not observed at shorter lengths…The finding that young microtubules are less probable to undergo catastrophe means that microtubules age: catastrophe frequency is not a constant, but rather increases with time.” (Gardner et al. 2013:14)

Journal article

Microtubule catastrophe and rescue

Current Opinion in Cell Biology, 25: 14–22 | 01/02/2013 | Gardner MK; Zanic M; Howard J

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Reference

Journal article

Microtubule assembly dynamics: new insights at the nanoscale

Current Opinion in Cell Biology, 20(1): 64-70 | 01/02/2008 | Gardner MK; Hunt AJ; Goodson HV; Odde DJ

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