The ubiquitin system

It has been often stated that until recently the ubiquitin system was thought to be mainly a 'garbage disposal' for the removal of abnormal or damaged proteins. This statement is certainly not true for those who have been interested in the selective and regulated degradation of proteins in cells. The dynamic turnover of cellular proteins was discovered in the pioneering studies of Rudolf Schoenheimer in the 1930s, when he first used isotopically labeled compounds for biological studies. Between 1960 and1970 it became evident that protein degradation in animal cells is highly selective, and is important in the control of specific enzyme concentrations. The molecular mechanisms responsible for this process, however, remained unknown. Some imaginative models have been proposed to account for the selectivity of protein degradation, such as one suggesting that all cellular proteins are rapidly engulfed into the lysosome, but only short-lived proteins are degraded in the lysosome, whereas long-lived proteins escape back to the cytosol.