Mapping Local and Global Liquid Phase Behavior in Living Cells Using Photo-Oligomerizable Seeds.

Liquid-liquid phase separation plays a key role in the assembly of diverse intracellular structures. However, the biophysical principles by which phase separation can be precisely localized within subregions of the cell are still largely unclear, particularly for low-abundance proteins. Here, we introduce an oligomerizing biomimetic system, "Corelets," and utilize its rapid and quantitative light-controlled tunability to map full intracellular phase diagrams, which dictate the concentrations at which phase separation occurs and the transition mechanism, in a protein sequence dependent manner. Surprisingly, both experiments and simulations show that while intracellular concentrations may be insufficient for global phase separation, sequestering protein ligands to slowly diffusing nucleation centers can move the cell into a different region of the phase diagram, resulting in localized phase separation. This diffusive capture mechanism liberates the cell from the constraints of global protein abundance and is likely exploited to pattern condensates associated with diverse biological processes. VIDEO ABSTRACT.

Asymmetric oligomerization state and sequence patterning can tune multiphase condensate miscibility.

Endogenous biomolecular condensates, composed of a multitude of proteins and RNAs, can organize into multiphasic structures with compositionally distinct phases. This multiphasic organization is generally understood to be critical for facilitating their proper biological function. However, the biophysical principles driving multiphase formation are not completely understood. Here we use in vivo condensate reconstitution experiments and coarse-grained molecular simulations to investigate how oligomerization and sequence interactions modulate multiphase organization in biomolecular condensates. We demonstrate that increasing the oligomerization state of an intrinsically disordered protein results in enhanced immiscibility and multiphase formation. Interestingly, we find that oligomerization tunes the miscibility of intrinsically disordered proteins in an asymmetric manner, with the effect being more pronounced when the intrinsically disordered protein, exhibiting stronger homotypic interactions, is oligomerized. Our findings suggest that oligomerization is a flexible biophysical mechanism that cells can exploit to tune the internal organization of biomolecular condensates and their associated biological functions.

Probing and engineering liquid-phase organelles.

Cells compartmentalize their intracellular environment to orchestrate countless simultaneous biochemical processes. Many intracellular tasks rely on membrane-less organelles, multicomponent condensates that assemble by liquid-liquid phase separation. A decade of intensive research has provided a basic understanding of the biomolecular driving forces underlying the form and function of such organelles. Here we review the technologies enabling these developments, along with approaches to designing spatiotemporally actuated organelles based on multivalent low-affinity interactions. With these recent advances, it is now becoming possible both to modulate the properties of native condensates and to engineer entirely new structures, with the potential for widespread biomedical and biotechnological applications.

Effect of a Flexible Linker on Recombinant Expression of Cell-Penetrating Peptide Fusion Proteins and Their Translocation into Fungal Cells.

Cell-penetrating peptides (CPPs) are a class of small peptides that are able to cross cell membranes via direct translocation or endocytosis. They have been widely used to deliver tethered bioactive molecules to cells, but recombinantly producing CPPs as fusions to protein cargo leads to low yields. We used Escherichia coli cells to recombinantly produce genetic fusions of NPFSD (derived from a yeast endocytosis signal) and pVEC (derived from a murine vascular endothelium cadherin) to the N-terminus of green fluorescent protein (GFP) with and without a flexible glycine-serine linker between the CPP and GFP. The flexible linker improved the expression of the NPFSD construct and the pVEC construct, resulting in a 24.5 % improvement in yield for the NPFSD fusion and a 50.0 % improvement in yield for the pVEC fusion. The linker did not diminish the ability of the fusions to translocate into the fungal pathogen Candida albicans, and the translocation of the NPFSD constructs actually increased by 58 % at 10 min. Moreover, the toxicity of the fusions towards C. albicans was not affected by the incorporation of the linker. These results illustrate the utility of including a linker for CPP-cargo fusions and the potential of NPFSD and pVEC fusions for use in delivering protein cargo to C. albicans.