Sperm cells “spring” into action

September 8, 2004

Tags: Protein Function

CAMBRIDGE, Mass. — Scientists have identified a surprising mechanical means by which cells store and release energy, a tightly wound jack-in-the-box mechanism rather than the chemical storehouse cells are known to use. This process, observed in the sperm cells of horseshoe crabs, was imaged at nearly atomic scale by researchers from Whitehead Institute for Biomedical Research and Baylor College of Medicine.

“This is nature’s prototype for how we can store energy and use it to do work,” says Paul Matsudaira, Whitehead Institute Member and co-author of the study, who believes these findings may also one day provide scientists with a new approach for building nanomachines.

That such an energy “prototype” should be discovered in sperm cells isn’t surprising. Sperm cells need to work hard to penetrate the protective coating surrounding an egg cell. Higher mammalian organisms typically do this by releasing enzymes that digest a small section of the egg’s membrane. The horseshoe crab, however, has a slightly more theatrical way of siring progeny. For these dome-shaped, pointed tail arthropods, sperm cells use molecular harpoons to spear through the egg’s membrane.

Scientists have witnessed this process and have also known that this harpoon comprises two proteins intertwined in a bundle of filaments. But they haven’t been able to determine exactly how the sperm cells garner the enormous energy to fire this spear. Now, using a high-power electron microscope, the research team has imaged this spear down to one nanometer—or one billionth of a meter—enabling them to figure out how it works.

Reporting in the September 2 issue of the journal Nature, the researchers describe, for the first time ever, the exact structure of this protein filament. Two molecules, actin and scruin, are tightly cross-linked with each other, forming a slightly irregular, almost distorted, helix. But this very irregularity is the secret behind the sperm cell’s power. The slight distortions in the helix cause an internal tension to build in the filament. When it is bundled inside the sperm cell at the base of the nucleus, it is like the spring inside of a jack-in-the-box with the lid fastened. Then, triggered by an environmental signal, the “latch” opens and the coil harpoons the egg.

“Normally actin works by a chemical means, but this process doesn’t require chemistry,” says Michael Schmid, associate professor of biochemistry at Baylor College of Medicine, and lead author of the study. “It just requires mechanics.”

The significance of this is twofold. First, actin is one of the most ubiquitous proteins in all species, including humans. Usually it’s responsible for such actions as cell division and muscle contraction. If actin filaments can store energy like this, then any kind of filament or polymer within a cell might also have the potential to mimic this energy storage trick. “Now that we’ve uncovered the principles behind the design, we can start looking at other biological processes,” says Matsudaira, who is also a professor of biology at MIT.

Matsudaira also is looking beyond basic cell biology and to the world of nanomachines, where scientists are trying to build instruments on the nanoscale for everything from computers to medical devices. In this field, biotechnology and other forms of engineering often overlap as researchers look to nature’s molecules for new ways to power these potentially microscopic machines.

“I can conceive of using this sort of cellular engine to power mechanical devices,” he says. “This finding tells us ways that we can think of powering nanomachines that we really hadn’t thought about before.”

Written by David Cameron.

Full citation
Nature, 430 (7004), pp. 104-107
“Structure of the acrosomal bundle”
Authors: Michael F. Schmid(1), Michael B. Sherman(1), Paul Matsudaira(2), and Wah Chiu(1)

(1) Baylor College of Medicine, One Baylor Plaza, Houston, Texas
(2) Whitehead Institute for Biomedical Research, Nine Cambridge Center, Cambridge, Massachusetts

This research is supported by the National Institutes of Health.

Horseshoe crab sperm

Nature's jack-in-the-box. The acrosomal bundle springs from the head of the horseshoe crab sperm cell by a purely mechanical means, ready to harpoon its way through an egg cell.

Photo: Jennifer Shin

Computer generated image of acrosomal bundles

Distorted helix. The image on the left, figure a, represents the traditional understanding of an acrosomal bundle structure. Scientists have discovered that the bundle’s structure isn’t as symmetrical as previously thought, shown in figure b.

Image: Michael Schmid and Matthew Dougherty, courtesy of Nature.


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