Structure and dynamics of hnRNP-labelled nuclear bodies induced by stress treatments

We have previously described HAP, a novel hnRNP protein that is identical both to SAF-B, a component of the nuclear scaffold, and to HET, a transcriptional regulator of the gene for heat shock protein 27. After heat shock, HAP is recruited to a few nuclear bodies. Here we report the characterisation of these bodies, which are distinct from other nuclear components such as coiled bodies and speckles. The formation of HAP bodies is part of a general cell response to stress agents, such as heat shock and cadmium sulfate, which also affect the distribution of hnRNP protein M. Electron microscopy demonstrates that in untreated cells, similar to other hnRNP proteins, HAP is associated to perichromatin fibrils. Instead, in heat shocked cells the protein is preferentially associated to clusters of perichromatin granules, which correspond to the HAP bodies observed in confocal microscopy. Inside such clusters, perichromatin granules eventually merge into a highly packaged 'core'. HAP and hnRNP M mark different districts of these structures. HAP is associated to perichromatin granules surrounding the core, while hnRNP M is mostly detected within the core. BrU incorporation experiments demonstrate that no transcription occurs within the stress-induced clusters of perichromatin granules, which are depots for RNAs synthesised both before and after heat shock.