HSP70 transgene directed motion to nuclear speckles facilitates heat shock activation

Association and disassociation of gene loci with respect to specific nuclear compartments accompany changes in gene expression, yet little is known concerning the mechanisms by which this occurs or its functional consequences. Previously, we showed that tethering acidic activators to a peripheral chromosome site led to movement of the chromosome site away from the nuclear periphery, but the physiological relevance of this movement was unclear [1]. Nuclear speckles, or interchromatin granule clusters, are enriched in factors involved in RNA processing [2], and the association of a subset of active genes at their periphery suggests speckles may play a role in gene expression [3, 4]. Here, we show an actin-dependent association of HSP70 transgenes with nuclear speckles after heat shock. We visualized HSP70 transgenes moving curvilinearly toward nuclear speckles over ∼0.5-6 μm distances at velocities of 1-2 μm min^-1. Chromatin stretching in the direction of movement demonstrates a force-generating mechanism. Transcription in nearly all cases increased noticeably only after initial contact with a nuclear speckle. Moreover, blocking new HSP70 transgene/speckle association by actin depolymerization prevented significant heat shock-induced transcriptional activation in transgenes not associated with speckles, although robust transcriptional activation was observed for HSP70 transgenes associated with nuclear speckles. Our results demonstrate the existence of a still-to-be-revealed machinery for moving chromatin in a direct path over long distances toward nuclear speckles in response to transcriptional activation; moreover, this speckle association enhances the heat shock activation of these HSP70 transgenes.