Harvard Medical School, Boston, MA 02115.
Mol Biol Cell. 2020 Nov 1;31(23):2502-2508. doi: 10.1091/mbc.E20-06-0393.
Macromolecule condensates, phase separation, and membraneless compartments have become an important area of cell biology research where new biophysical concepts are emerging. This article discusses the possibility that condensates assemble on multivalent surfaces such as DNA, microtubules, or lipid bilayers by multilayer adsorption. Langmuir isotherm theory conceptualized saturable surface binding and deeply influenced physical biochemistry. Brunauer-Emmett-Teller (BET) theory extended Langmuir's ideas to multilayer adsorption. A BET-inspired biochemical model predicts that surface-binding proteins with a tendency to self-associate will form multilayered condensates on binding surfaces. These "bound condensates" are expected to assemble well below the saturation concentration for liquid-liquid phase separation, so they can compete subunits away from phase-separated droplets and are thermodynamically pinned to the binding surface. Tau binding to microtubules is an interesting test case. The nonsaturable binding isotherm is reminiscent of BET predictions, but assembly of Tau-rich domains at low concentrations requires a different model. Surface-bound condensates may find multiple biological uses, particularly in situations where it is important that condensate assembly is spatially constrained, such as gene regulation.
大分子凝聚物、相分离和无膜隔间已成为细胞生物学研究的一个重要领域，新的生物物理概念正在涌现。本文讨论了凝聚物是否可以通过多层吸附在多价表面（如 DNA、微管或脂质双层）上组装的可能性。朗缪尔等温线理论构想了可饱和的表面结合，并深刻影响了物理生物化学。布劳尔-埃梅特-泰勒（BET）理论将朗缪尔的思想扩展到多层吸附。受 BET 启发的生化模型预测，具有自我聚集倾向的表面结合蛋白将在结合表面上形成多层凝聚物。这些“结合凝聚物”预计将在低于液-液相分离的饱和浓度下组装，因此它们可以将亚基从相分离的液滴中竞争出来，并在热力学上固定在结合表面上。Tau 与微管的结合是一个有趣的测试案例。不可饱和的结合等温线让人联想到 BET 的预测，但在低浓度下富含 Tau 的结构域的组装需要一个不同的模型。表面结合的凝聚物可能有多种生物学用途，特别是在凝聚物组装受到空间限制的情况下，例如基因调控。
