Rattle and hum: molecular machinery makes yeast cells purr.

It's not just heart cells that beat; yeast cells also move to their own rhythm. While investigating the physical properties of yeast cells, researchers at the University of California, Los Angeles (UCLA) discovered that the cell walls were vibrating. They propose that it was the hum of the cells' internal machinery.

The UCLA team detected the vibrations while prodding the cells with an atomic force microscope. The instrument, used to analyze nanoscale structures, consists of a microscopic cantilever with a down-pointing needle sharpened to just a few atoms wide.

The researchers placed the cantilever so that its tip rested gently on the surface of a living yeast cell. The cantilever tip moved up and down over a few nanometers at a frequency of 1.6 kilohertz. This indicated a repeated expansion and contraction of the cell wall, the researchers say in the Aug. 20 Science.

To determine what was causing this nanomotion, the researchers measured the vibration of the cells at various temperatures between 22[degrees]C and 30[degrees]C. As the temperature increased, the frequency of vibration increased, suggesting that a metabolic process was governing the motion. When the researchers exposed the cells to sodium azide--a metabolic inhibitor--the motion all but died out.

The UCLA team also translated the cell's recorded motion into a digital sound file, enabling the researchers to listen to the cell under various conditions. "It's like taking a gramophone needle, but instead of the record rotating, the cell goes up and down," says lead investigator James Gimzewski.

Sounds of the cells can be heard at http://www.chem.ucla.edu/dept/Faculty/gi mzewski/teapot/. The recordings illustrate that the pitch changes with temperature.

Because the mechanical properties of the cell wall remained constant but experiments yielded different frequencies, Gimzewski says the vibration couldn't result from the wall's natural resonance. By the UCLA team's calculations, molecular-motor proteins inside the cell are the likely source of the rumble. Such proteins carry chemical cargo along molecular tracks called microtubules and pump nutrients in and out of cells.

George Oster, a molecular biologist at the University of California, Berkeley, describes the findings as "very interesting.: However, he offers another scenario to explain the vibration. Cells are riddled with channels that open and close in response to environmental cues. If the cantilever tip pressed down on one of these channels, it might have made the cell repeatedly shrink and swell.

Previously, researchers had used an atomic force microscope to measure the natural beating of heart cells. However, observing regular vibrations in other cells is novel, says Gimzewski. Analyzing cells on the basis of how they pulsate could be a new approach to understanding disease.

Gimzewski's lab is currently using the phenomenon to study cancer. Cancerous cells are softer than normal cells, so they might vibrate at different frequencies than normal cells do. Vibration frequency might someday serve as a marker for disease, says Gimzewski.

This concept is both cutting edge and a bit old-fashioned. "It's like going back to the early days of medicine before they had all these blood tests, and the doctor just listened to your heart," Gimzewski says. In the future, the doctor might instead listen to your cells.