Progress on Crowding Effect in Cell-like Structures

Abstract

Several biological macromolecules, such as proteins, nucleic acids, and polysaccharides, occupy about 30% of the space in cells, resulting in a crowded macromolecule environment. The crowding effect within cells exerts an impact on the functions of biological components, the assembly behavior of biomacromolecules, and the thermodynamics and kinetics of metabolic reactions. Cell-like structures provide confined and independent compartments for studying the working mechanisms of cells, which can be used to study the physiological functions arising from the crowding effect of macromolecules in cells. This article mainly summarizes the progress of research on the macromolecular crowding effects in cell-like structures. It includes the effects of this crowding on actin assembly behavior, tubulin aggregation behavior, and gene expression. The challenges and future trends in this field are presented at the end of the paper.

Keywords: actin assembly behavior; cell-like structures; crowding effect; gene expression; tubulin aggregation behavior.

Figure 1

(a) Rendering of the E. coli cytoplasm model, in which different-colored shapes represent different macromolecules, including GFP, tRNA, etc. Reprinted with permission from Ref. [7]. 2010, McGuffee, Elcock. Crowding effect illustration in a cell-like structure containing large red spheres and small purple spheres before crowding (b) and after crowding (c).

Figure 2

(a) The crowding effect of macromolecules promoted actin-ring formation at the equatorial plane of the droplet. Reprinted with permission from Ref. [36] 2015, Springer Nature. (b) FtsZ bundle formation with no crowder, polymeric crowder PEG-8000, protein crowder BSA, and E. coli lysate. Reprinted with permission from Ref. [37] 2015, American Chemical Society. (c) Repeated deformation of spindle-shaped liposomes encapsulating high-concentration actin. Reprinted with permission from Ref. [38] 2018, Springer Nature. (d) MreB produced by cell-free expression deformed liposomes with membranes composed of PC and PE-PEG. Reprinted with permission from Ref. [39] 2020, American Chemical Society.

Figure 3

(a) Tubulin polymerization inside Tau droplets. Reprinted with permission from Ref. [41] 2018, Elsevier. (b) Tubulins nucleation inside TPX2 droplets to allow the formation of microtubules. Reprinted with permission from Ref. [42] 2020, Springer Nature. (c) Tubulin nucleation inside SPD-5/TPXL-1/ZYG-9 condensates. Reprinted with permission from Ref. [43] 2017, Elsevier.

Figure 4

(a) A spontaneous concentration of all components of the mixture inside vesicles by spontaneous formation of liposomes, enabling protein synthesis. Reprinted with permission from Ref. [45] 2013, John Wiley and Sons. (b) Inhomogeneous distribution of mRNA over one droplet at high ficoll concentrations. Reprinted with permission from Ref. [46] 2015, Springer Nature. (c) Enhancement of gene expression in artificial cells by crowding effect. Reprinted with permission from Ref. [47] 2013, Springer Nature.