Quote of the Week
(December 4, 2023)

It has become increasingly clear in recent years that, quite apart from its cytoskeleton and membrane-bound organelles, the fluid cytoplasm in each cell is elaborately and “invisibly” organized. Various macromolecular complexes and other molecules, in more or less defined mixes, congregate in specific locations and sustain a collective identity, despite being unbounded by any sort of membrane. Here we’re looking at significant structure, or organization, without even a pretense of mechanically rigid form. How do cells manage that?

The problem was framed this way by Anthony Hyman from the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, and Clifford Brangwynne from the Department of Chemical and Biological Engineering at Princeton University:

“Non-membrane-bound macromolecular assemblies found throughout the cytoplasm and nucleoplasm ... consist of large numbers of interacting macromolecular complexes and act as reaction centers or storage compartments ... We have little idea how these compartments are organized. What are the rules that ensure that defined sets of proteins cluster in the same place in the cytoplasm?”

Even more puzzling, a “compartment” can maintain its functional (purposive) identity despite the rapid exchange of its contents with the surrounding cytoplasm. Hyman and Brangwynne ask: “Fast turnover rates of complexes in compartments can be found throughout the cell. How do these remain as coherent structures when their components completely turn over so quickly?”

(from Chapter 5, “Our Bodies Are Formed Streams”, in Organisms and Their Evolution — Agency and Meaning in the Drama of Life)

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