Post-translational modification of proteasome subunits

Proteasome and RC subunits are subjected to a variety of post-translational modifications including phosphorylation, acetylation, myristoylation, and even O-glycosylation [136-140]. In yeast all seven a-subunits are acetylated as well as two yS-subunits. Since acetylation of the N-terminal threonine in an active j8-subunit would poison catalysis, it has been suggested that the propeptide extensions function to prevent acetylation [130]. Three members of the S4 ATPase subfamily (S4, 

and SlOb) and two 20S a-subunits (C8 and C9) are known to be phosphoryl-ated. Phosphorylation appears to be particularly important for 26S proteasome assembly and stability. The kinase inhibitor staurosporine reduces 26S proteasome levels in mouse lymphoma cells [135] and interferon y results in reduced phosphorylation of 20S proteasome a-subunits and decreased 26S proteasome levels [141]. 

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