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Secrets of the cell transport system

AFP-JIJI

Vesicles, the bubble-shaped vessels that transport molecules within cells, may hold the secret to halting viruses or even combating Alzheimer’s, say experts who hailed Monday’s Nobel awarded to three leaders in the field.

Thousands of these saclike cargo vehicles reside in every human cell.

Their job is to transport molecules within and between cells.

Vesicles are crucial for the very functioning of cells — shipping everything from neurotransmitter molecules responsible for signaling in the brain to hormones and immune responses to viral intruders.

When things go well, the vesicles are like trucks providing just-in-time delivery of parts for a factory buzzing with activity.

But if vesicles arrive too late or too early, or go to the wrong place, they can cause the cell to malfunction — a potential trigger for disease. They also allow viruses to propagate by transporting viral proteins within cells.

“Very fundamental processes in life depend on vesicular transport,” said Dick Hoekstra, cell biology professor at the University of Groningen in the Netherlands.

“All organelles (compartments) within a cell are communicating with each other via vesicles,” he said. “The way that viruses reproduce is also very much dependent on the vesicular transport system.”

Inside cells is a hive of machinery that cranks out proteins, the molecules that sustain the cell and enable it to function, but also help it work with other cells.

Most molecules produced by cells are too large to pass through cell membranes and thus are transported instead by vesicles.

The vesicles are coated with fatty membranes that “fuse” with the membranes on other vesicles, to pass on their cargo, a bit like water drops merging on a windowpane.

They are able to identify their destination thanks to an “address code” of proteins studding the membrane. The proteins are like the matching sides of a zip: When they fit, two vesicles merge and the cargo is transferred.

“Its almost like a bubble hitting a wall and the contents of that bubble are then released,” explained Mike Cousins, chairman of neuronal cell biology at Edinburgh University.

Each vesicle is about 50 nanometers (billionths of a meter) in diameter and transfers about a million molecules at a time. Vesicles continually “bud off” from one cell membrane, bulging out to fuse to another, then releasing their precious, molecular cargo.

The three men honored by the Nobel Assembly on Monday, U.S. duo James Rothman and Randy Schekman and German-born Thomas Suedhof, have put decades of work into shedding more light on how vesicles work — and why, sometimes, they do not.

Schekman figured out which genes were responsible for mixing up the transport system, which can cause vesicles to pile up in parts of the cell like a traffic jam. Rothman discovered that a protein complex enables vesicles to dock and fuse. Suedhof took the work further by showing more precisely how the protein “zipper” opens up to allow neurotransmitters to move between nerve cells in the brain.

The knowledge has shed light on the important role vesicles may be playing in diseases like diabetes and immune disorders.