Abstract: Proteasomes play a key regulatory role in all eukaryotic cells by removing proteins in a timely manner. There are two predominant forms: The 20S core particle (CP) can hydrolyze peptides and certain unstructured proteins, and the 26S holoenzyme is able to proteolyse most proteins conjugated to ubiquitin (Ub). The 26S complex consists of a CP barrel with a 19S regulatory particle (RP; a.k.a PA700) attached to its outer surface. A number of studies purified another proteasome activator with a MW of 200 kDa (PA200) that attaches to the same outer ring of the CP. A role for PA200 has been demonstrated in spermatogenesis, in response to DNA repair and in maintenance of mitochondrial inheritance. Enhanced levels of PA200-CP complexes are observed under conditions in which either activated or disrupted CP prevail, suggesting it participates in regulating overall proteolytic activity. PA200, or its yeast ortholog Blm10, may also incorporate into 26S proteasomes yielding PA200-CP-RP hybrids. A 3D molecular structure determined by X Ray crystallography of Blm10-CP provides a model for activation. The Carboxy-terminus of Blm10 inserts into a dedicated pocket in the outer ring of the CP surface, while multiple HEAT-like repeats fold into an asymmetric solenoid wrapping around the central pore to stabilize a partially open conformation. The resulting hollow dome-like structure caps the entire CP surface. This asymmetric structure may provide insight as to how the 19S RP, with two HEAT-repeat like subunits (Rpn1, Rpn2) alongside six ATPases (Rpt1-6), attaches to the same surface of the CP ring, and likewise, induces pore opening.