A Protein-Processing Program Plays A Role In Cell Signaling

"There might be a whole array of events occurring at the cell surface or at internal membranes that are regulated in this way," says Raphael Kopan, Ph.D., associate professor of medicine and molecular biology & pharmacology at Washington University School of Medicine in St. Louis. Kopan led a study on the processing of the developmental protein Notch that will be published in today's Molecular Cell. His findings corroborate those of scientists who have studied proteins that regulate lipid metabolism, the folding of new proteins and other processes.

In a May 1998 article in Nature, Kopan and his colleagues revealed that a fragment of Notch serves as a messenger that determines a cell's fate. Notch, a protein found inserted in the outer membrane of cells, has a part that juts out in the cell's surroundings and another that juts into the interior of the cell. The part that sticks out serves as a receptor and attaches to a protein ligand presented by nearby cells.

The researchers determined that this interaction permits the release of the internal portion of Notch, which travels to the nucleus to influence gene activity so that a cell becomes one of two cell types, such as a neuron or an epidermal cell.

Notch's tail fragment is thought to be released from the cell surface in a process involving a protein that also helps cleave the tail of an Alzheimer's disease-related protein. This protein, amyloid precursor protein (APP), gives rise to a smaller fragment that forms plaques in the brains of patients with the disease.

The study in Molecular Cell reveals that Notch sheds its outer portion when it attaches to other cells. Kopan and his colleagues have given the name NEXT (Notch extracellular truncation) to the remainder.

He postulates that Notch unfurls itself when it binds to ligand, making the outer portion susceptible to cleavage. This cleavage creates NEXT, which Kopan’s study shows is the version of Notch from which the inner tail is released.

Kopan compares unbound Notch to a mousetrap that is ready to spring into action once a mouse steps on the bait. "Notch at the surface is cocked and ready to go," he says. "If a ligand binds to it, a series of proteolytic cleavages releases Notch's business end — the intracellular fragment that goes to the nucleus to create a change (in cell behavior)."

Other investigators in the same issue of Molecular Cell identified the enzyme that cleaves the outer portion of Notch and APP. In combination, the papers' results suggest that Alzheimer researchers should more closely evaluate APP to determine whether it receives signals from nearby cells, Kopan says. In the past two years, scientists have identified five proteins in addition to Notch and APP that undergo cleavage at membranes.

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Mumm JS, Schroeter EH, Saxena MT, Tian X, Griesemer A, Pan DJ, Ray WJ, Kopan R. Ligand induced "ectodomain shedding" regulates gamma-secretase-like proteolytic activation of Notch1. Molecular Cell, 5 (2): 197-206, Feb. 25, 2000.

A review of protein processing at membranes: Brown MS, Ye J, Rawson RB, Goldstein JL. Regulated Intramembrane Proteolysis: A Control Mechanism Conserved from Bacteria to Humans. Cell ,100 (4): 391-398, Feb. 18, 2000.

This research was funded by a grant from the National Institute of General and Medical Sciences and by a Lucille P. Markey Pathway fellowship.