Lot6p - a redox regulated switch of the proteasome
During our previous studies of bacterial quinone reductases, we have also investigated the biochemical properties of the yeast homolog Lot6p. Despite the availability of a three-dimensional structure for Lot6p (1T0I), the physiological role of the enzyme was unclear.
Our recent studies have now demonstrated that the enzyme rapidly reduces quinones at the expense of a reduced nicotinamide cofactor, either NADH or NADPH. In order to further characterize the cellular role of Lot6p, we have carried out pull-down assays and identified the 20S core particle of the yeast proteasome as interaction partner. Further studies revealed that this complex recruits Yap4p, a member of the b-Zip transcription factor family, but only when the flavin-cofactor of Lot6p is in its reduced state. Oxidation of the flavin leads to dissociation of the transcription factor and relocalization to the nucleus (see scheme below).
A similar system is known from mammalian cells, where a homologous quinone reductase (NQO1) binds to the 20S proteasome and recruits important tumor suppressor proteins such as p53 and p73?. Hence, the discovery of a homologous protein interaction in yeast provides an interesting model system to investigate the molecular basis for protein complex formation and regulation of proteasomal degradation of transcription factors (thesis project of Wolf-Dieter Lienhart and Venugopal Gudipati, master thesis of Karin Koch).