In Arabidopsis thaliana more than 1300 genes (~5% of the proteome) encode components of the ubiquitin/26S proteasome pathway. Approximately 90% of these genes encode subunits of the E3 ubiquitin ligases, which confer substrate specificity to the ubiquitin/26S proteasome pathway. The plant E3 ubiquitin ligases comprise a large and diverse family of proteins or protein complexes containing a RING-finger, U-box domain or a HECT domain.

The article gives basic information on proteasome- and ubiquitin-dependent system of pro- tein degradation. The ubiquitination process of proteins designated for proteasomal breakdown and ubiquitin involvement in extraproteasomal cellular events is discussed. The article explains the struc- ture of 20S, 26S and PA700 complexes, the role of immunoproteasomes in the immune response and proteasomal proteolysis independent of ubiquitination. Moreover, it gives the outline of proteasomal biogenesis and possible clinical application of proteasome inhibitors.