Feel the vibe: optical investigation of the interfacial properties of biomolecular condensates

Interactions between proteins and nucleic acids (NAs) are crucial for countless cellular functions. In particular, NAs can regulate solubility, concentration and activity of proteins through the formation of membraneless organelles, or biomolecular condensates, arising from spontaneous liquid-liquid phase separation [1]. This mechanism is increasingly recognized as a major principle for spatial and temporal organization in cells, from gene expression [2] to DNA repair and stress response [3], but can also be associated to the onset of pathological protein aggregation [4]. The physiological role of condensates is inherently related to material properties like viscosity, elasticity and surface tension and it is thus of critical importance to quantify such properties with effective and non-invasive methods [5].

Here, we apply Differential Dynamic Microscopy (DDM) [6] to probe the dynamics of individual condensate droplets sedimented on a solid substrate, by analyzing the spectrum of intensity fluctuations in bright-field microscopy images. While previous work extracted bulk properties of condensates [7], we demonstrate that we can gain label-free access to the amplitude and relaxation dynamics of thermally-driven surface fluctuations at the interface between the condensed and dilute phases. Furthermore, from the combination of DDM with Fluorescence Recovery After Photobleaching and coalescence experiments, we can separate viscosity and surface tension contributions. We determine their dependence on sequence, structure and ionic strength in various in vitro models of biomolecular condensates, including mixtures of NAs with ribosomal peptides [8].



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[2] Henninger et al. Cell 184,207 (2021)

[3] Pessina et al. Nat. Cell Biol. 21,1286 (2019)

[4] Alberti & Carra JMB 430,4711 (2018)

[5] Michieletto & Marenda JACS Au 2,1506 (2022)

[6] Cerbino & Trappe PRL 100,188102 (2008)

[7] Linsenmeier et al. Nat. Commun. 13,3030 (2022)

[8] Codispoti et al. NAR 52,12689 (2024)