An active mechanism flanks and modulates the export of the small ribosomal subunits.

The modalities of export of the ribosomal subunits from the nucleolus to the nuclear pores have been only partially clarified since it is not yet clear whether the movements depend purely on diffusion or also from an active process. Recently, we suggested the existence of an active transport mechanism of a subset (10-12%) of the small ribosomal subunits (SSU) (Cisterna et al. in 2006, Faseb J). Here, we give further evidence that an active, motor protein-mediated process exists for the SSU transport from the nucleolus to the nuclear pore. We demonstrate that the blockade of ATP synthesis and antibody-mediated inhibition of nuclear myosin or actin induce structural and functional modifications of the nucleolus, suggestive of transcriptional activity decrease. Moreover, both treatments induce a significant retention of RNA inside the nucleus and an accumulation of ribosomal subunits in the granular component. We suggest that the existence of this secondary, active mechanism of SSU transport might be utilized by the cell when a more rapid and directional export is needed.