Researchers at the University of Chicago have found that a recently discovered biological process known as sumoylation -- until now thought to be active only in the nucleus -- also occurs near the cell's surface where it regulates at least one and possibly many kinds of proteins, providing a novel target for the development of new drugs.
The discovery, published in the 8 April 2005 issue of the journal Cell, answers a question dating back to the 1950s: How do cells control the background movement of potassium ions across the cell membrane? This process is important because the flow of potassium ions determines whether "excitable" cells in the brain, heart and skeletal muscles "fire," sending out nervous impulses that become thoughts, heartbeats and basketball dunks.
"We found that a little-studied process called sumoylation, previously associated with nuclear proteins, is active and essential outside the nucleus at the plasma membrane," said study author Steven Goldstein, M.D., Ph.D., professor and chairman of pediatrics and director of the Institute for Molecular Pediatric Sciences at the University of Chicago. "This adds a new chapter to the book of how cells control ion channel function: reversible peptide linkage."
Ion channels are in every cell in the human body. They are tightly controlled tunnels through the membrane barriers that hold in the cell's contents, separating the cell from the outside world. Ion channels allow ions such as potassium, sodium and calcium to flow in and out and so are key regulators of many fundamental processes in biology.
"Ions are the currency of the cellular world," explained Goldstein. "Cells collect some ions, others they discharge. Ions are stored, spent, and exchanged."
"Cellular solvency," he added, "the ability to respond to the stimuli that are life, is all about the balance between ions inside and outside each cell. The gradual doling out or sudden influx of ions through ion channels are the basis for those cellular activities that give us thoughts, sights, tastes, sounds and our ability to move."
"Consequently," he adds, "cells control these actions as carefully as we watch our finances, which is why so many of the most potent mediations we use to care for our patients' target one or another ion channel."
Goldstein's team discovered the type of ion channel known as background (or leak) potassium channels in yeast cells in 1995 and in fruit flies in 1996. Although potassium leak was first described in the 1950s when it was recognized to control excitation of nerves, the reason for leak had not previously been understood.
The first human clone of this channel, K2P1, generated a good deal of excitement, Goldstein said, but no one could learn much about it because it always seemed to be mute. "This discouraged a lot of people."
The problem was that some hidden mechanism was silencing the channel, plugging the pipeline, but no known method of channel regulation seemed to be involved.
Goldstein and colleagues began to suspect sumoylation [SUE-mow-e-LAY-shun]. In this process, an enzyme attaches a small peptide called SUMO (for small ubiquitin-like modifier protein) onto another protein. The presence of SUMO alters how the second protein functions.