Exit Strategy

Large RNA-protein packets use a novel mechanism to escape the cell nucleus.

By | November 1, 2012




BYPASSING THE PORE: Once thought to be the only method of exit for RNA, nuclear pores (pictured here) are no longer the only way out.© DON W. FAWCETT/SCIENCE SOURCE

The paper
S.D. Speese et al., “Nuclear envelope budding enables large ribonucleoprotein particle export during synaptic Wnt signaling,” Cell, 149:832-46, 2012.

The finding
While studying synapse development, Vivian Budnik, of the University of Massachusetts Medical School, and colleagues noticed large granules of RNA and protein in the nuclei of post-synaptic muscle cells. These particles were larger than the nuclear pore—the only known exit portal for RNA. With the help of RNA expert Melissa Moore, also at U Mass, the researchers determined that the granules exited by budding through the inner and outer nuclear membranes.
The pathway
Budnik had been tracking the contribution of a Wnt signaling protein called Wingless (Wg) to synapse development. Using a range of stains, the team discovered that a fragment of the Wg receptor recruits RNA encoding postsynaptic proteins and becomes associated with a large ribonucleoprotein granule. The granule then exits the nucleus by first becoming enveloped in the inner nuclear membrane, then fusing with the outer nuclear membrane, releasing the naked granule into the cytoplasm.

The copycat
A very similar budding process had been observed in herpes virus export from the cell nucleus. However, the mechanism was thought to be a rare example of a virus creating a novel cell process rather than hijacking an endogenous one.

The package
Budnik thinks the granules may contain a “complete care package” with all the information for constructing the postsynaptic apparatus. “It opens the door for so many next sets of experiments addressing mechanism,” says Susan Wente of Vanderbilt University, who studies the nuclear pore complex and was not involved in the study.

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