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Same here: we tried using mScarlet-i3 in a CRY2 optogenetic assay (blue-light stimulus!) to track membrane recruitment of a Scarlet-tagged protein, but ran into unexpectedly strong photobleaching.

Implications? 🤔new conceptual framework for understanding ER compartmentalisation, the regulation of the sheet-tubule equilibrium, and organelle plasticity? 🤔homeoviscous mechanism - rods acting as a sink for solidifying lipids to buffer the ER against acute thermal or metabolic fluctuations? /7

How do rods form? Solid domains exclude reticulon proteins, which require packing defects for membrane insertion. Their displacement inhibits ER tubulation, driving membrane flattening and multilayered wrapping into rods. 🦉The ER exploits membrane solidification to spatially organize proteins! /6

Mammalian membranes are thought to universally avoid macroscopic solidification, as reduced fluidity impairs plasticity and protein function. Cells counter this via lipid unsaturation and cholesterol buffering. However, the ER is deliberately cholesterol-poor to maintain high-curvature membranes!/5

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Rods form as a function of temperature and lipid composition: in standard cell culture typically <30°C but also in vivo at 37°C - in alveolar type II lung cells natively enriched in saturated lipids /4 🙏Melissa Mikolaj @ Kedar Narayan lab

Rods have a continuous multilamellar architecture - unlike any known cellular membrane organisation - and occur across all cell types tested /3 🙏 @andimicroscopy.bsky.social

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We show that - in mammalian cells and in vivo - saturated lipids in the #endoplasmic reticulum can demix into stable, micrometer-scale #solid-like domains. The resulting long-range orientational order in the membrane gives rise to cell-spanning, rigid tubular structures, we term #rods /2

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