Heterogeneity in the kinetics of nuclear proteins and trajectories of substructures associated with heterochromatin

We show that protein dynamics are influenced by many factors and events, including nuclear pattern and transcription activity. A slower recovery after photobleaching was found when proteins, such as HP1β, BMI1, TRF1, and others accumulated at specific foci. In identical cells, proteins that were evenly dispersed throughout the nucleoplasm recovered more rapidly. Distinct trajectories for HP1β, BMI1, and TRF1 were observed after hyperacetylation or suppression of transcription. The relationship between protein trajectory and transcription level was confirmed for telomeric protein TRF1, but not for HP1β or BMI1 proteins. Moreover, heterogeneity of foci movement was especially observed when we made distinctions between centrally and peripherally positioned foci.Based on our results, we propose that protein kinetics are likely influenced by several factors, including chromatin condensation, differentiation, local protein density, protein binding efficiency, and nuclear pattern. These factors and events likely cooperate to dictate the mobility of particular proteins.The eukaryotic nucleus is highly compartmentalised. The functional consequences of nuclear compartmentalisation have been described previously in both fixed and live cells [1-3]. Generally, chromatin consists of histones wrapped with DNA and the many proteins that are directly responsible for proper nuclear functions, such as replication, transcription, splicing, and DNA repair. Nuclear processes proceed in specific compartments, such as nuclear foci, transcription and replication factories, or nuclear speckles (summarised in [4,5]). Moreover, interphase chromosomes are arranged into chromosome territories, which can intermingle to some degree [4,6,7]. Centromeres and telomeres are also essential structures for chromosome function. Centromeres are the sites of mitotic spindle attachment and are required for cell division [8]. Telomeres, which include the shelterin proteins (telomeric-repeat binding factor 1 (