The Two PAN ATPases from Halobacterium Display N-terminal Heterogeneity and Form Labile Complexes with the 20S Proteasome

The PAN (proteasome-activating nucleotidase) proteins from archaea represent homologues of the eukaryotic 26S proteasome regulatory ATPases. In vitro the PAN complex has been previously shown to have a stimulatory effect on the peptidase activities of the 20S core. By using gradient ultracentrifugation we found that, in cellular extracts, the two PAN proteins from Halobacterium do not form stable high-molecular-mass complexes. Only PAN B was found to associate transiently with the 20S proteasome, thus suggesting that the two PAN proteins are not functionally redundant. The PAN B-20S proteasome complexes associate in an ATP-dependent manner and are stabilized upon nucleotide binding. The two PAN proteins were immunodetected in cellular extracts as N-terminal-truncated polypeptides. RNA-mapping experiments and sequence analysis indicated that this process involved transcript heterogeneities and dual translational initiation mechanisms. Taken together, our results suggest that PAN N-terminal modifications and their intracellular dynamics of assembly/association may constitute important determinants of proteolysis regulation.