Co-translational stabilization of insoluble proteins in cell-free expression systems

High-throughput cell-free screening of eukaryotic membrane protein expression in lipidic mimetics

Membrane proteins play essential roles in cellular function and metabolism. Nonetheless, biophysical and structural studies of membrane proteins are impeded by the difficulty of their expression in and purification from heterologous cell-based systems. As an alternative to these cell-based systems, cell-free protein synthesis has proven to be an exquisite method for screening membrane protein targets in a variety of lipidic mimetics. Here we report a high-throughput screening workflow and apply it to screen 61 eukaryotic membrane protein targets. For each target, we tested its expression in lipidic mimetics: two detergents, two liposomes, and two nanodiscs. We show that 35 membrane proteins (57%) can be expressed in a soluble fraction in at least one of the mimetics with the two detergents performing significantly better than nanodiscs and liposomes, in that order. Using the established cell-free workflow, we studied the production and biophysical assays for mitochondrial pyruvate carrier (MPC) complexes. Our studies show that the complexes produced in cell-free are functionally competent in complex formation and substrate binding. Our results highlight the utility of using cell-free systems for screening and production of eukaryotic membrane proteins.

Functional Analysis of Aquaporin Water Permeability Using an Escherichia coli-Based Cell-Free Protein Synthesis System

Aquaporins are essential water channel proteins found in all kingdoms of life. Although the water permeability of aquaporins has been well characterized, sample preparation for aquaporin water permeability assays remains challenging and time-consuming. Besides the difficulty in overexpressing membrane proteins in a cell-based expression system, the unique requirement for homogeneity in aquaporin proteoliposome sample preparations for water transport assays further increases the complexity. In this study, a complementary Cell-free Protein Synthesis (CFPS) method is described in detail, providing three different strategies for the preparation of aquaporin proteoliposome samples. Aquaporin can be produced either as a pellet fraction and then resolubilized, or co-translationally as a detergent-soluble fraction. Furthermore, aquaporin can be directly incorporated into liposomes, which was included in the CFPS reactions. Although proteoliposomes tend to fuse during the incubation of the CFPS reactions, an additional treatment of the fused samples with detergent, followed by a detergent removal step, can re-form homogenously sized proteoliposomes suitable for functional analysis. Using this method, we successfully characterized aquaporins from both prokaryotic and eukaryotic organisms. In particular, in the presence of liposomes, the developed CFPS expression system is a fast and convenient method for sample preparation for the functional analysis of aquaporins.

Cell-Free Synthetic Glycobiology: Designing and Engineering Glycomolecules Outside of Living Cells

Glycans and glycosylated biomolecules are directly involved in almost every biological process as well as the etiology of most major diseases. Hence, glycoscience knowledge is essential to efforts aimed at addressing fundamental challenges in understanding and improving human health, protecting the environment and enhancing energy security, and developing renewable and sustainable resources that can serve as the source of next-generation materials. While much progress has been made, there remains an urgent need for new tools that can overexpress structurally uniform glycans and glycoconjugates in the quantities needed for characterization and that can be used to mechanistically dissect the enzymatic reactions and multi-enzyme assembly lines that promote their construction. To address this technology gap, cell-free synthetic glycobiology has emerged as a simplified and highly modular framework to investigate, prototype, and engineer pathways for glycan biosynthesis and biomolecule glycosylation outside the confines of living cells. From nucleotide sugars to complex glycoproteins, we summarize here recent efforts that harness the power of cell-free approaches to design, build, test, and utilize glyco-enzyme reaction networks that produce desired glycomolecules in a predictable and controllable manner. We also highlight novel cell-free methods for shedding light on poorly understood aspects of diverse glycosylation processes and engineering these processes toward desired outcomes. Taken together, cell-free synthetic glycobiology represents a promising set of tools and techniques for accelerating basic glycoscience research (e.g., deciphering the "glycan code") and its application (e.g., biomanufacturing high-value glycomolecules on demand).

Co-Translational Insertion of Aquaporins into Liposome for Functional Analysis via an E. coli Based Cell-Free Protein Synthesis System

Aquaporins are important and well-studied water channel membrane proteins. However, being membrane proteins, sample preparation for functional analysis is tedious and time-consuming. In this paper, we report a new approach for the co-translational insertion of two aquaporins from Escherichia coli and Nicotiana tabacum using the CFPS system. This was done in the presence of liposomes with a modified procedure to form homogenous proteo-liposomes suitable for functional analysis of water permeability using stopped-flow spectrophotometry. Two model aquaporins, AqpZ and NtPIP2;1, were successfully incorporated into the liposome in their active forms. Shifted green fluorescent protein was fused to the C-terminal part of AqpZ to monitor its insertion and status in the lipid environment. This new fast approach offers a fast and straightforward method for the functional analysis of aquaporins in both prokaryotic and eukaryotic organisms.

Recent advances in development of cell-free protein synthesis systems for fast and efficient production of recombinant proteins

Cell-free protein synthesis has emerged in recent years as a powerful tool that can potentially transform the production of recombinant proteins. Cell-free protein synthesis harnesses the synthetic power of living cells while eliminating many of the constraints of traditional cell-based gene expression methods. Due to the lack of physical barriers separating the protein synthesis machinery from the surrounding environment, a cell-free protein synthesis reaction mixture can be directly programmed using diverse genetic material for the instant production of recombinant proteins without complicated cloning procedures. However, a number of issues must be addressed for this technology to be widely accepted as an alternative platform for protein production, including quality-control of translation machinery preparations, and high reagent cost. This review describes recent efforts to make cell-free protein synthesis more affordable and more easily accessible for generic applications.

A synthetic biology platform for the reconstitution and mechanistic dissection of LINC complex assembly

The linker of nucleoskeleton and cytoskeleton (LINC) is a conserved nuclear envelope-spanning molecular bridge that is responsible for the mechanical integration of the nucleus with the cytoskeleton. LINC complexes are formed by a transluminal interaction between the outer and inner nuclear membrane KASH and SUN proteins, respectively. Despite recent structural insights, our mechanistic understanding of LINC complex assembly remains limited by the lack of an experimental system for its in vitro reconstitution and manipulation. Here, we describe artificial nuclear membranes (ANMs) as a synthetic biology platform based on mammalian cell-free expression for the rapid reconstitution of SUN proteins in supported lipid bilayers. We demonstrate that SUN1 and SUN2 are oriented in ANMs with solvent-exposed C-terminal KASH-binding SUN domains. We also find that SUN2 possesses a single transmembrane domain, while SUN1 possesses three. Finally, SUN protein-containing ANMs bind synthetic KASH peptides, thereby reconstituting the LINC complex core. This work represents the first in vitro reconstitution of KASH-binding SUN proteins in supported lipid bilayers using cell-free expression, which will be invaluable for testing proposed models of LINC complex assembly and its regulation.

Potential role of protein stabilizers in amelioration of Parkinson's disease and associated effects in transgenic Caenorhabditis elegans model expressing alpha-synuclein

Studies employing transgenicC. elegansmodel show that trehalose, a protein stabilizer, alleviates manifestations associated with Parkinson's diseaseviaits inherent activity and through induction of autophagic machinery.