THE UBIQUITIN SYSTEM IN PLANTS

The small polypeptide ubiquitin participates in a variety of fundament al cellular events, such as cell differentiation, stress response. det ermination of steady state levels of regulatory proteins, cell cycle c ontrol, regulation of transcription, and programmed cell death. Althou gh the complex mechanisms of these processes are not fully understood, ubiquitinylation of regulatory proteins involved in those events is o bviously essential. Target proteins can be covalently coupled with one or a few ubiquitin molecules, which is supposed to present a (reversi ble) post-translational modification. Polyubiquitinylation, however, m arks proteins selectively for degradation by the 26S proteasome. The u biquitin system has been studied mostly with animal systems or yeast, but all basic reactions of ubiquitin appear in plants as well. The sco pe of this review is to summarize several implications of recent studi es directed towards the plant ubiquitin system.