Biologists have discovered that certain proteins vital to life and long thought to be immobilized within the sack-like cell structures where they function instead move freely and rapidly within the structures.
The discovery contradicts widely accepted models for the function of the structures, or organelles, called the Golgi apparatus. There is no obvious explanation for why, despite their movement, the enzymes are not transported out of the organelle, as other mobile proteins are.
Usually, proteins moving within the organelle are eventually packaged in vesicles -- small sacks of membrane -- pinched off and then transported out to the rest of the cell, where they are needed for a variety of purposes.
But these specific proteins are not transported; they are mysteriously retained in the organelle. Without the retention, the organelles would not function properly and vital life processes would break down.
Scientists had proposed that the proteins are anchored and immobilized in the organelle, so that they could perform their essential functions.
The new findings, however, show just the opposite.
"They are milling all around," said Michael Edidin, a Johns Hopkins University biologist involved in the research.
The findings are detailed in a paper published Aug. 9 in the journal Science. And the rapid protein motion can be viewed in a video clip to be posted on the Internet, at the following address: http://www.uchc.edu/htterasaki/flip.html
The researchers were able to study the movement of proteins through a revolutionary technique that uses light-emitting, or fluorescent, molecules taken from a jellyfish, Aequorea victoria. DNA from the jellyfish's fluorescent protein is fused with the DNA of proteins biologists want to study, making them fluorescent.
Then, researchers used special laser-operated microscopes to follow the movement of the proteins inside the cell.
The Golgi apparatus processes and packages molecules needed for a multitude of functions. The biologists were surprised to discover that the proteins move fast enough to traverse the organelle within a few seconds. At that speed, they should easily be taken up by vesicles and transported outside of the Golgi apparatus.
"How Golgi membranes maintain their identity amidst this rapid diffusion of resident components now needs to be explained," said the senior author of the paper, Jennifer Lippincott-Schwartz, from the National Institutes of Health.
Other biologists involved in the work were Nelson B. Cole and Carolyn Smith of NIH, Mark Terasaki from the University of Connecticut and Noah Sciaky from the National Jewish Center for Immunology and Respiratory Medicine.