On Press:
AP-2 Not So Essential After All

By Jason Socrates Bardi

Sometimes an accessory you thought was absolutely necessary turns out not to be necessary after all.

Imagine that you thought that city buses were the only way to get across town. The one thing you think you need is your bus pass. Without your pass, how could you ride? But then you lose your bus pass. You discover you can pay with loose change. Or you can ride the subway. Or take taxis; pedicabs; foot paths; even trolley cars. It turns out that the bus pass is not so essential.

The findings published by two scientists at The Scripps Research Institute (TSRI) this month in the Journal of Cell Biology are somewhat analogous. In the report, Sean Conner, a research associate, and Sandra Schmid, professor and chair of the Department of Cell Biology, demonstrate that clathrin-mediated endocytosis can still take place even when a protein that was thought absolutely essential for the process is lost.

Clathrin-mediated endocytosis is a cellular process whereby important hormones, proteins, nutrients, and other macromolecular "cargo" needed by a cell are collected into small transport vehicles called 'coated vesicles' and carried into the cell. Coated vesicles are derived from patches of membrane where receptors are located and where the cargo molecules are gathered. That patch of membrane becomes involuted, bulging inward to form a pit that is surrounded on the inside of the cell by a lattice-like coat of protein known as clathrin. The involuted membrane pinches off, forming a clathrin-coated vesicle filled with cargo.

Adaptor protein complex molecules (AP-2) are key components in clathrin-mediated endocytosis because they trigger clathrin assembly, interact directly with cargo molecules, and recruit a number of other accessory factors. AP-2 molecules were believed essential for clathrin-mediated endocytosis.

In their paper, however, Conner and Schmid disrupted the function of AP-2 molecules by overexpressing an enzyme called the adaptor-associated kinase (AAK1), which phosphorylates AP-2 and modulates its function. Too much AAK1 means not enough AP-2, and they assumed that limiting AP-2 would negatively affect clathrin-mediated endocytosis. To their surprise, while some cargo molecules were no longer taken up by the cell, the clathrin-mediated uptake of other cargo molecules was unaffected.

This work suggests the AP-2 molecule may not be stoichiometrically required for coat assembly, as previously thought, and that AP-2 molecules may be just one class of adaptor proteins that function to couple clathrin coat assembly with the efficient packaging of cargo molecules.

To read the article, "Differential requirements for AP-2 in clathrin-mediated endocytosis" by Sean D. Conner and Sandra L. Schmid, please see the Journal of Cell Biology, 162, 773-780 or go to http://www.jcb.org/cgi/content/abstract/162/5/773.




Adaptor proteins drive the assembly of clathrin coats at the plasma membrane and coordinate the selective packaging of receptor/ligand complexes into clathrin-coated vesicles for internalization. Click to enlarge.