Small ribosomal subunits associate with nuclear myosin and actin in transit to the nuclear pores.

We have followed at high resolution theribosomal protein S6 entering the nucleus of HeLacells, stopping in some (not all) interchromatin granulesclusters and reaching, via Cajal bodies, the nucleolus.There, S6 is assembled with other proteins andrRNA into small ribosomal subunit (SSU), released inthe nucleoplasm, and exported through the nuclearpores. We show for the first time the spatial associationof nuclear myosin I (NMI) and actin with the SSUalready at the nucleolar periphery to the nuclear pore.A blockade of NMI or actin induces an upstreamaccumulation of the S6 protein en route to the nucleolus,and a temperature lower than normal influencesRNA export. Our data strongly suggest a functionalrelationship of SSU with NMI and actin. In our hypothesis,an active, myosin-driven movement of the smallribosomal subunit can be responsible for the exportof 10% of SSUs. This hypothesis is supported byultrastructural, immunofluorescence, and biochemicalanalyses. The currently accepted model for the subunitrelease suggests a diffusive, temperature-independentmechanism. However, the advantage of the doublemechanism would assure that the movement of a partof the subunits could be modulated, increased, ordecreased according to the needs of the cell at a specificmoment in the cell cycle.—Cisterna, B., Necchi, D.,Prosperi, E., Biggiogera, M. Small ribosomal subunitsassociate with nuclear myosin and actin in transit to thenuclear pores.