The proteasome, the central protease of eukaryotic cells, is composed of one core particle (CP) and one or two adjacent regulatory particles (RP), which contain multiple subunits. Several proteasome-dedicated chaperones govern the assembly of CP and RP, respectively. We sought for proteins that regulate final steps of RP-CP assembly in yeast and found Ecm29, a conserved HEAT-like repeat protein. Here, we show that Ecm29 controls the integrity of RP-CP assemblies. Ecm29 recognizes RP-CP species in which CP maturation is stalled due to the lack of distinct beta subunits. Reconstitution assays revealed that Ecm29 functions as scaffold protein during the remodeling of incompletely matured RP-CP assemblies into regular enzymes. Upon the completion of CP maturation, Ecm29 is degraded and RP-CP is dissociated.
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