Figure 1: Ubiquitin-proteasome pathway and schematic of the cellular proteasome. The ubiquitin-proteasome pathway tags proteins with chains of ubiquitin to be degraded in the catalytic core of the proteasome. The ubiquitin E1 enzyme binds ubiquitin and transports it to the E2 enzyme. The E2 enzyme then transports the ubiquitin to the target substrate via the E3 ubiquitin ligase. Substrates (proteins) that are polyubiquitinated are degraded by the proteasome. Proteasomes regulate the concentration of particular proteins and degrade misfolded proteins. The proteasome is composed of the catalytic core 20S complex, which comprises a- and ß-subunits, and two 19S regulatory complexes. Together with ATPases, these form the 26S proteasome. The degradation of the cellular proteins is a vital step in the regulation of various signal transduction pathways. The degradation process yields peptides which can then be further degraded into amino acids and used in synthesizing new proteins.