Route of glucocorticoid-induced macromolecules across the nuclear envelope as viewed by atomic force microscopy.

Glucocorticoids are vital steroid hormones. The physiologic activities of these hydrophobic molecules predominantly require translocation of glucocorticoid-initiated macromolecules (GIMs), proteins and mRNA transcripts, in and out of the nucleus, respectively. The bidirectional transport of GIMs is mediated by nuclear pore complexes (NPCs) that span the nuclear envelope at regular distances. The transport proceeds through the NPC central channel, whose interior is lined up by hydrophobic proteins. The NPC channel is assumed to dilate while hydrophobic cargos are being translocated through. Upon glucocorticoid injection into a glucocorticoid-sensitive cell, Xenopus laevis oocyte, and using atomic force microscopy, we have recently unraveled the long unexplored paths that GIMs take through the nuclear envelope and described interactions of GIMs with NPCs. In so doing, surprising and intriguing observations were made and the following conclusions were drawn: glucocorticoid-initiated proteins evoke NPC channel dilation before physical interaction with the NPC. NPC channel dilation is apparently transmitted through binding of glucocorticoid-induced proteins to NPC-associated filaments or yet unknown structures in the cytoplasmic nuclear envelope surface. The transport of both proteins and ribonucleoproteins seems to be non-randomly confined to local areas on either nuclear envelope site, the so-called hot spots.