Curated Optogenetic Publication Database

Abstract: Mitochondria contain double membranes that enclose their contents. Within their interior, the mitochondrial genome and its RNA products are condensed into ∼100 nm sized (ribo)nucleoprotein complexes. How these endogenous condensates maintain their roughly uniform size and spatial distributions within membranous mitochondria remains unclear. Here, we engineered an optogenetic tool (mt-optoIDR) that allowed for controlled formation of synthetic condensates upon light activation in live mitochondria. Using live cell super-resolution microscopy, we visualized the nucleation of small, yet elongated condensates (mt-opto-condensates), which recapitulated the morphologies of endogenous mitochondrial condensates. We decoupled the contribution of the double membranes from the environment within the matrix by overexpressing the dominant negative mutant of a membrane fusion protein (Drp1K38A). The resulting bulbous mitochondria had significantly more dynamic condensates that coarsened into a single, prominent droplet. These observations inform how mitochondrial membranes can limit the growth and dynamics of the condensates they enclose, without the need of additional regulatory mechanisms.

Abstract: The nucleolus is a prominent nuclear condensate that plays a central role in ribosome biogenesis by facilitating the transcription and processing of nascent ribosomal RNA (rRNA). A number of studies have highlighted the active viscoelastic nature of the nucleolus, whose material properties and phase behavior are a consequence of underlying molecular interactions. However, the ways in which the material properties of the nucleolus impact its function in rRNA biogenesis are not understood. Here we utilize the Cry2olig optogenetic system to modulate the viscoelastic properties of the nucleolus. We show that above a threshold concentration of Cry2olig protein, the nucleolus can be gelled into a tightly linked, low mobility meshwork. Gelled nucleoli no longer coalesce and relax into spheres but nonetheless permit continued internal molecular mobility of small proteins. These changes in nucleolar material properties manifest in specific alterations in rRNA processing steps, including a buildup of larger rRNA precursors and a depletion of smaller rRNA precursors. We propose that the flux of processed rRNA may be actively tuned by the cell through modulating nucleolar material properties, which suggests the potential of materials-based approaches for therapeutic intervention in ribosomopathies.