The lysosome is the "digestive system" of an animal cell. Molecules taken up from the outside are sent to the lysosome to be broken up into a form that can be safely used by the rest of the cell. A network of membrane vesicles called the endocytic pathway moves cargo destined for the lysosome from the surface of the cell. One of the last steps before the cargo reaches the cell is the pinching-off of small vesicles into the center of a big vesicle. When the big vesicles join the lysosome by fusion, the small vesicles inside it are released into the lysosome and broken down by powerful degradative enzymes. The stage where small vesicles reside inside the bigger vesicle is called a "multivesicular body" (MVB). The machinery for making MVBs is hijacked by the HIV virus to escape from cells and is considered a potential target for HIV therapeutics. In its normal functioning, the MVB machinery breaks down proteins that can cause cancer when they are present in excess. Using x-ray diffraction data collected at the APS and SSRL Beam Line 9-2, collaborators from the NIH and the University of California at San Diego have determined the first structure of an ESCRT complex.

Nature. 2004 Sep 9; 431(2004):221-5