Proteasomes

Three major cellular mechanisms are involved in the breakdown of proteins (proteolysis). One of these is by the previously mentioned endosomal-lysosomal degradation. Another cytoplasmic degradation mechanism is the cas-pase pathway that that is involved in apoptotic cell death (see Chapter 5). The third method of proteolysis occurs within an organelle called the proteasome. Proteasomes are quite large (approximately 2000 kd) and are thought to be present in the nucleus and the cytoplasm. This organelle recognizes misformed and misfolded proteins that are des­tined for degradation, including transcription factors and the cyclins that are important in controlling the cell cycle. It has been suggested that as many as one third of the newly formed polypeptide chains are selected for protea­some degradation due to quality control mechanisms within the cell.