The 26S-proteasome: regulation and substrate recognition.

There is extensive reprogramming of the ATPase regulators of the 26S proteasome before the programmed elimination of the abdominal intersegmental muscles (ISM) after eclosion in Manduca sexta [1]. This extensive ATPase reprogramming only occurs in ISM which are destined to die and not in flight muscle (FM). The MS73 ATPase also increases in the proleg retractor muscles which die at a developmentally different stage to ISM. The non-ATPase regulator S5a shows a similar increase to the ATPase regulators. We have cloned the Manduca SUG2 ATPase and shown that this ATPase is a component of the 26S proteasome. This ATPase shows a similar increase in concentration to the other ATPases in 26S proteasomes before muscle death. The SUG2 ATPase is also associated with other smaller complexes besides the 26S proteasome which act as activators of the 26S proteasome. Finally, in a yeast two-hybrid genetic screen we have identified a protein in human brain which interacts with the MS73 ATPase (and human S6). The interacting protein contains 6 ankyrin repeats and is co-immunoprecipitated with anti-MS73 antiserum after in vitro transcription/translation. The ankyrin repeat protein may interact with the MS73 ATPase as part of the substrate recognition process by the 26S proteasome. Many proteins degraded by the 26S proteasome contain ankyrin repeats, e.g. IkB and some cyclins: binding through ankyrin repeats to an ATPase regulator may complement protein ubiquitination and S5a binding as recognition signals by the 26S proteasome.