FRAP reveals that mRNPs (white) move faster than diffusion alone allows.

The nucleus is a crowded place, with chromatin, nuclear speckles, and nucleoli clogging up the works. Although the usual crop of molecular motors have not been found in the nucleus, many nuclear molecules are able to move from place to place. Recently, diffusion has been argued to be the principal means of nuclear travel. But new results from Calapez et al. (page 795) argue that Brownian motion does not account for everything. Something is using energy to help large particles move in the nucleus.

Large particles that must traverse the nucleus include mRNPs, a complex of mRNA and several associated protein factors. Calapez et al. analyzed mRNP movement from chromatin to the nuclear pore by FRAP analysis of two mRNA-binding proteins tagged with GFP. mRNP complexes moved more quickly than dextrans of the same size, which are expected to move by passive diffusion. So, mRNPs are getting help navigating the nucleus.

The identity of the movement aide is not clear, but it does consume energy. ATP depletion or a reduction in temperature slowed mRNPs. One promising candidate is the nuclear ATP-dependent RNA helicase Dbp5p, which is required for mRNA export. It is not known, however, whether it acts at or before the nuclear pore. The authors are currently using RNA interference to assess the involvement of Dbp5p. As yet, there is no evidence for directional movement of mRNPs, as might occur along a filamentous network. Rather, the authors suggest that the energy-dependent process frees an mRNP particle from obstacles it encounters, such as chromatin, allowing it to diffuse without hindrance. ▪