The unphosphorylated form of the PAQosome core subunit RPAP3 binds ribosomal preassembly complexes to modulate ribosome biogenesis.

The PAQosome (Particle for Arrangement of Quaternary structure) is a twelve-subunit HSP90 co-chaperone involved in the biogenesis of several human protein complexes. Two mechanisms of client selection have previously been identified, namely the selective recruitment of specific adaptors and the differential use of homologous core subunits. Here, we describe a third client selection mechanism by showing that RPAP3, one of the core PAQosome subunits, is phosphorylated at several Ser residues in HEK293 cells. Affinity purification coupled with mass spectrometry (AP-MS) using expression of tagged RPAP3 with single phospho-null mutations at Ser116, Ser119 or Ser121 reveals binding of the unphosphorylated form to several proteins involved in ribosome biogenesis. In vitro phosphorylation assays indicate that the kinase CK2 phosphorylates these RPAP3 residues. This finding is supported by data showing that pharmacological inhibition of CK2 enhances binding of RPAP3 to ribosome preassembly factors in AP-MS experiments. Moreover, silencing of PAQosome subunits interferes with ribosomal assembly factors interactome. Altogether, these results indicate that RPAP3 phosphate group addition/removal at specific residues modulates binding to subunits of preribosomal complexes and allows speculating that PAQosome posttranslational modifications is a mechanism of client selection.