Organelles and spatial organization of the cell: organelle homeostasis and turnover

CONSTRUCTING UNUSUAL ORGANELLES Two talks described mechanisms for the formation of unusual organelles. The talk by Elias Cornejo (Komeili laboratory, University of California, Berkeley) described the remodeling of bacterial membrane to form a series of evenly spaced invaginations. Iron oxide crystallizes into magnetic particles inside these membrane invaginations. The spacing and formation of the membrane invaginations do not depend on iron oxide crystallization; however, the size of the membrane invagination appears to facilitate the crystal formation process. If a crystal is not made, the membrane invagination will stall at a diameter of ∼50 nm. Once crystal formation has initiated, the membrane invagination can expand to significantly larger sizes and accommodate mature magnetic particles. In another talk, Avinash Patel (Hyman laboratory, Max-PlanckInstitute, Dresden, Germany) showed results from biochemical experiments aimed at dissecting the mechanisms of liquid-to-solid phase transition of the ALS protein FUS. This protein normally has a disordered domain that can loosely associate with similar proteins to form a non–membrane-bounded compartment with liquid properties. These liquids exhibit phase separation from the surrounding aqueous environment akin to the separation of oil and water droplets. Given enough time, the concentrated protein solution can also convert to fibrous aggregates. This conversion to aggregates is accelerated by mutations associated with ALS, which leads to the intriguing suggestion that aberrant phase transitions are key to ALS and other age-related diseases.