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

We have followed at high resolution the ribosomal protein S6 entering the nucleus of HeLa cells, stopping in some (not all) interchromatin granules clusters and reaching, via Cajal bodies, the nucleolus. There, S6 is assembled with other proteins and rRNA into small ribosomal subunit (SSU), released in the nucleoplasm, and exported through the nuclear pores. We show for the first time the spatial association of nuclear myosin I (NMI) and actin with the SSU already at the nucleolar periphery to the nuclear pore. A blockade of NMI or actin induces an upstream accumulation of the S6 protein en route to the nucleolus, and a temperature lower than normal influences RNA export. Our data strongly suggest a functional relationship of SSU with NMI and actin. In our hypothesis, an active, myosin-driven movement of the small ribosomal subunit can be responsible for the export of approximately 10% of SSUs. This hypothesis is supported by ultrastructural, immunofluorescence, and biochemical analyses. The currently accepted model for the subunit release suggests a diffusive, temperature-independent mechanism. However, the advantage of the double mechanism would assure that the movement of a part of the subunits could be modulated, increased, or decreased according to the needs of the cell at a specific moment in the cell cycle.