Coppey-Moisan, M. | bioRxiv Science

bioRxiv2018

Chromatin Condensation Fluctuations Rather Than Steady-State Predict Chromatin Accessibility

Chromatin accessibility to protein factors is critical for genome activities. Dynamic changes in nucleosomal DNA compaction and higher order chromatin structures are expected to allow specific sites to be accessible to regulatory factors and the transcriptional machinery. However, the dynamic properties of chromatin that regulate its accessibility are poorly understood. Here, we took advantage of the microenvironment sensitivity of the fluorescence lifetime of EGFP-H4 histone incorporated in chromatin to map in the nucleus of live cells the dynamics of chromatin condensation and its direct interaction with a tail acetylation recognition domain (the double bromodomain module of human TAFII250, dBD). We reveal chromatin condensation fluctuations supported by mechanisms fundamentally distinct from that of condensation. Fluctuations are spontaneous, yet their amplitudes are affected by their sub-nuclear localization and by distinct and competing mechanisms dependent on histone acetylation, ATP, and both. Moreover, we show that accessibility of acetylated histone H4 to dBD is not restricted by chromatin condensation nor predicted by acetylation, rather, it is predicted by chromatin condensation fluctuations.\n\nSignificanceIn higher eukaryotes, the structure and compaction of chromatin are considered as barriers to genome activities. Epigenetic marks such as post-translational modifications of histones can modify the structure and compaction of chromatin. The accessibility of protein factors to these epigenetic marks is therefore of paramount importance for genome activities. We reveal chromatin condensation fluctuations supported by mechanisms fundamentally distinct from that of condensation itself. We show that accessibility of acetylated histone H4 to double bromodomains is not restricted by chromatin condensation nor predicted by acetylation, rather, it is predicted by chromatin condensation fluctuations.\n\nClassificationBiological Sciences, Cell Biology

cell biology↗

bioRxiv2017

Biomechanical control of lysosomal secretion via the VAMP7 hub: a tug-of-1 war mechanism between VARP and LRRK1

The rigidity of the cell environment can vary tremendously between tissues and in pathological conditions. How this property may affect intracellular membrane dynamics is still largely unknown. Here, using atomic force microscopy, we found that cells deficient in the secretory lysosome v-SNARE VAMP7 were impaired in adapting to substrate rigidity. Conversely VAMP7-mediated secretion was stimulated by more rigid substrate and this regulation depended on the Longin domain of VAMP7. We further found that the Longin domain bound the kinase and retrograde trafficking adaptor LRRK1 and LRRK1 negatively regulated VAMP7-mediated exocytosis. Conversely, VARP, a VAMP7- and kinesin 1-interacting protein, further controlled the availability for secretion of peripheral VAMP7 vesicles and response of cells to mechanical constraints. We propose a mechanism whereby biomechanical constraints regulate VAMP7- dependent lysosomal secretion via LRRK1 and VARP tug-of-war control of the peripheral readily- releasable pool of secretory lysosomes.

cell biology↗