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On the Origin of Non-Membrane-Bound Organelles, and Their Physiological Function


On the Origin of Non-Membrane-Bound Organelles, and Their Physiological Function

Wylie Stroberg et al. J Theor Biol.


The origin of cellular compartmentalization has long been viewed as paralleling the origin of life. Historically, membrane-bound organelles have been presented as the canonical examples of compartmentalization. However, recent interest in cellular compartments that lack encompassing membranes has forced biologists to reexamine the form and function of cellular organization. The intracellular environment is now known to be full of transient macromolecular structures that are essential to cellular function, especially in relation to RNA regulation. Here we discuss key findings regarding the physicochemical principles governing the formation and function of non-membrane-bound organelles. Particularly, we focus how the physiological function of non-membrane-bound organelles depends on their molecular structure. We also present a potential mechanism for the formation of non-membrane-bound organelles. We conclude with suggestions for future inquiry into the diversity of roles played by non-membrane bound organelles.

Keywords: Cellular biophysics; Endosymbiosis; Liquid-liquid phase separation; Macromolecular crowding; Non-membrane-bound organelles.


Figure 1
Figure 1. Length scales of non-membrane-bound organelles
The sizes of non-membrane-bound organelles span a large range, from tens of nanometers in the case of ribosomes, to microns in the cases of oocyte nucleoli (Brangwynne et al., 2011). The majority of non-membrane-bound organelles are hundreds of nanometers in size, placing them at an intermediate length scale between macromolecules and most membrane-bound organelles.
Figure 2
Figure 2. Phase-separated regulation of chaperone-assisted folding
The top panel shows three possible mixtures of a protein, which can be in an unfolded conformation A, or folded conformation A*, and chaperone B that catalyzes the transition from A to A*. The bottom panel displays schematic progress curves for the folding of A, corresponding to the mixtures directly above. The left column shows a well-mixed case, in which the reaction proceeds at a moderate pace. In the middle column, the mixture separates into a phase rich in proteins in the unfolded conformation and chaperones, catalyzing the transition of A to A*. In the right column, the mixture forms a phase devoid of chaperones, effectively sequestering proteins in conformation A.

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