Localisation and organisation of the focal adhesion kinase FAK1 and Simiate inside nuclear speckles

The development of a well-organised genome represents a hallmark in the evolution of species. In mammals, the nucleus of each cell is characterised by the presence of different compartments, among others nuclear speckles, membrane-less organelles that are self-shaped by liquid droplet-like phase separation. Functioning in the organisation of the transcription and splicing machinery, nuclear speckles are highly dynamic, moving and rearranging within the nucleus according to the needs of the cell. In line with a role of actin dynamics in speckle function, we could previously demonstrate that the actin-binding protein Simiate is not only enriched in nuclear speckles, but also able to associate with nuclear isoforms of the Focal Adhesion Kinase FAK1. Furthermore, nuclear speckles have recently been suggested to consist of specific sub-domains involved in the spatial organisation of chromatin handling and mRNA processing. In this study, we therefore examined the sub-speckular organisation of FAK1 and Simiate in mouse brain slices using three-dimensional reconstructions and stimulated emission depletion (STED) microscopy. While FAK1 is predominantly localised in peripheral areas, Simiate is highly enriched in the core domain. Aside, Simiate is also seen in the surrounding shell, and minor amounts of FAK1 are detected in the core domain. As the number of speckles increases, FAK1 is found to diminish from the core domain, whereas peripheral numbers remain constant. Both proteins, Simiate and FAK1, are organised in spherical clusters, which may occasionally colocalise in peripheral as well as core domains. Although our data obtained from mouse brain slices are merely descriptive, they may suggest for dynamic rearrangement of FAK1.