Garbage collectors are important for removing trash; without them waste accumulates and can quickly become a health hazard.
Similarly, individual cells that make up such biological organisms as humans also have sophisticated methods for managing waste.
For example, cells have developed complex systems for recycling, reusing and disposing of damaged, nonfunctional waste proteins. When such systems malfunction and these proteins accumulate, they can become toxic, resulting in many diseases, including Alzheimer's, cystic fibrosis and developmental disorders.
Scott Emr, director of the Weill Institute for Cell and Molecular Biology at Cornell, and colleagues, describe in detail how cells recycle protein waste in two recent papers appearing in the journals Cell and Developmental Cell .
"We are interested in understanding how cells deal with garbage," said Emr. "It's really a very sophisticated recycling system."
Cells use enzymes known as proteases to break down proteins into their component amino acids in the cytoplasm -- the fluid inside the cell's surface membrane. Those amino acids are then reused to make new proteins. But water-insoluble proteins embedded in the cell's membrane require a much more complicated recycling process.
Emr's paper in Cell identifies a family of proteins that controls the removal of unwanted water-insoluble proteins from the membrane. The research advances understanding of how cells recognize which proteins out of hundreds on a cell's surface should be removed. For example, hormone receptors at a cell's surface signal such processes within the cell as growth and proliferation. To inactivate these receptors and terminate the growth signal, receptors are tagged for removal. Failure to inactivate can lead to developmental diseases and cancer.
The researchers, including postdoctoral fellows Jason MacGurn and Chris Stefan, identified nine related proteins in yeast, which they named the "arrestin-related trafficking" adaptors or ARTs. Each of these proteins identifies and binds to a different set of membrane proteins. Once bound, the ART protein links to an enzyme that attaches a chemical tag for that protein's removal. The ARTs are found in both yeast and humans, suggesting the fundamental nature of their function.