Unphosphorylated Form of the PAQosome Core Subunit RPAP3Binds Ribosomal Preassembly Complexes to Modulate Ribosome Biogenesis br

The PAQosome (particle for arrangement of quaternarystructure) is a 12-subunit HSP90 co-chaperone involved in thebiogenesis of several human protein complexes. Two mechanisms ofclient selection have previously been identified, namely, the selectiverecruitment of specific adaptors and the differential use of homologouscore subunits. Here, we describe a third client selection mechanism byshowing that RPAP3, one of the core PAQosome subunits, isphosphorylated at several Ser residues in HEK293 cells. Affinitypurification coupled with mass spectrometry (AP-MS) using theexpression of tagged RPAP3 with single phospho-null mutations atSer116, Ser119, or Ser121 reveals binding of the unphosphorylatedform to several proteins involved in ribosome biogenesis.In vitrophosphorylation assays indicate that the kinase CK2 phosphorylatesthese RPAP3 residues. Thisfinding is supported by data showing that pharmacological inhibition of CK2 enhances the binding ofRPAP3 to ribosome preassembly factors in AP-MS experiments. Moreover, the silencing of PAQosome subunits interferes withribosomal assembly factors'interactome. Altogether, these results indicate that RPAP3 phosphate group addition/removal at specificresidues modulates binding to subunits of preribosomal complexes and allows speculating that PAQosome posttranslationalmodification is a mechanism of client selection.