Kinetic frustration by limited bond availability controls the LAT protein condensation phase transition on membranes

Transmembrane signaling through single-spanning receptors modulated by phase separation at the cell surface

A wide variety of transmembrane signals are transduced by cell-surface receptors that activate intracellular signaling molecules. In particular, receptor clustering in the plasma membrane plays a critical role in these processes. Single-spanning or single-pass transmembrane proteins are among the most significant types of membrane receptors, which include adhesion receptors, such as integrins, CD44, cadherins, and receptor tyrosine kinases. Elucidating the molecular mechanisms underlying the regulation of the activity of these receptors is of great significance. Liquid-liquid phase separation (LLPS) is a recently emerging paradigm in cellular physiology for the ubiquitous regulation of the spatiotemporal dynamics of various signaling pathways. This study describes the emerging features of transmembrane signaling through single-spanning receptors from the perspective of phase separation. Possible physicochemical modulations of LLPS-based transmembrane signaling are also discussed.

Mechanical forces amplify TCR mechanotransduction in T cell activation and function

Adoptive T cell immunotherapies, including engineered T cell receptor (eTCR) and chimeric antigen receptor (CAR) T cell immunotherapies, have shown efficacy in treating a subset of hematologic malignancies, exhibit promise in solid tumors, and have many other potential applications, such as in fibrosis, autoimmunity, and regenerative medicine. While immunoengineering has focused on designing biomaterials to present biochemical cues to manipulate T cells ex vivo and in vivo, mechanical cues that regulate their biology have been largely underappreciated. This review highlights the contributions of mechanical force to several receptor-ligand interactions critical to T cell function, with central focus on the TCR-peptide-loaded major histocompatibility complex (pMHC). We then emphasize the role of mechanical forces in (i) allosteric strengthening of the TCR-pMHC interaction in amplifying ligand discrimination during T cell antigen recognition prior to activation and (ii) T cell interactions with the extracellular matrix. We then describe approaches to design eTCRs, CARs, and biomaterials to exploit TCR mechanosensitivity in order to potentiate T cell manufacturing and function in adoptive T cell immunotherapy.

Spontaneous transfer of small peripheral peptides between supported lipid bilayer and giant unilamellar vesicles

Vesicular trafficking facilitates material transport between membrane-bound organelles. Membrane protein cargos are trafficked for relocation, recycling, and degradation during various physiological processes. In vitro fusion studies utilized synthetic lipid membranes to study the molecular mechanisms of vesicular trafficking and to develop synthetic materials mimicking the biological membrane trafficking. Various fusogenic conditions which can induce vesicular fusion have been used to establish synthetic systems that can mimic biological systems. Despite these efforts, the mechanisms underlying vesicular trafficking of membrane proteins remain limited and robust in vitro methods that can construct synthetic trafficking systems for membrane proteins between large membranes (>1 μm2) are unavailable. Here, we provide data to show the spontaneous transfer of small membrane-bound peptides (∼4 kD) between a supported lipid bilayer (SLB) and giant unilamellar vesicles (GUVs). We found that the contact between the SLB and GUVs led to the occasional but notable transfer of membrane-bound peptides in a physiological saline buffer condition (pH 7.4, 150 mM NaCl). Quantitative and dynamic time-lapse analyses suggested that the observed exchange occurred through the formation of hemi-fusion stalks between the SLB and GUVs. Larger protein cargos with a size of ∼77 kD could not be transferred between the SLB and GUVs, suggesting that the larger-sized cargos limited diffusion across the hemi-fusion stalk, which was predicted to have a highly curved structure. Compositional study showed Ni-chelated lipid head group was the essential component catalyzing the process. Our system serves as an example synthetic platform that enables the investigation of small-peptide trafficking between synthetic membranes and reveals hemi-fused lipid bridge formation as a mechanism of peptide transfer.

Two-dimensional molecular condensation in cell signaling and mechanosensing

Membraneless organelles (MLO) regulate diverse biological processes in a spatiotemporally controlled manner spanning from inside to outside of the cells. The plasma membrane (PM) at the cell surface serves as a central platform for forming multi-component signaling hubs that sense mechanical and chemical cues during physiological and pathological conditions. During signal transduction, the assembly and formation of membrane-bound MLO are dynamically tunable depending on the physicochemical properties of the surrounding environment and partitioning biomolecules. Biomechanical properties of MLO-associated membrane structures can control the microenvironment for biomolecular interactions and assembly. Lipid-protein complex interactions determine the catalytic region's assembly pattern and assembly rate and, thereby, the amplitude of activities. In this review, we will focus on how cell surface microenvironments, including membrane curvature, surface topology and tension, lipid-phase separation, and adhesion force, guide the assembly of PM-associated MLO for cell signal transductions.

Self-programmed dynamics of T cell receptor condensation

A common event upon receptor-ligand engagement is the formation of receptor clusters on the cell surface, in which signaling molecules are specifically recruited or excluded to form signaling hubs to regulate cellular events. These clusters are often transient and can be disassembled to terminate signaling. Despite the general relevance of dynamic receptor clustering in cell signaling, the regulatory mechanism underlying the dynamics is still poorly understood. As a major antigen receptor in the immune system, T cell receptors (TCR) form spatiotemporally dynamic clusters to mediate robust yet temporal signaling to induce adaptive immune responses. Here we identify a phase separation mechanism controlling dynamic TCR clustering and signaling. The TCR signaling component CD3ε chain can condensate with Lck kinase through phase separation to form TCR signalosomes for active antigen signaling. Lck-mediated CD3ε phosphorylation, however, switched its binding preference to Csk, a functional suppressor of Lck, to cause the dissolvement of TCR signalosomes. Modulating TCR/Lck condensation by targeting CD3ε interactions with Lck or Csk directly affects T cell activation and function, highlighting the importance of the phase separation mechanism. The self-programmed condensation and dissolvement is thus a built-in mechanism of TCR signaling and might be relevant to other receptors.

Effects of linker length on phase separation: lessons from the Rubisco-EPYC1 system of the algal pyrenoid

Biomolecular condensates are membraneless organelles formed via phase separation of macromolecules, typically consisting of bond-forming "stickers" connected by flexible "linkers". Linkers have diverse roles, such as occupying space and facilitating interactions. To understand how linker length relative to other lengths affects condensation, we focus on the pyrenoid, which enhances photosynthesis in green algae. Specifically, we apply coarse-grained simulations and analytical theory to the pyrenoid proteins of Chlamydomonas reinhardtii: the rigid holoenzyme Rubisco and its flexible partner EPYC1. Remarkably, halving EPYC1 linker lengths decreases critical concentrations by ten-fold. We attribute this difference to the molecular "fit" between EPYC1 and Rubisco. Varying Rubisco sticker locations reveals that the native sites yield the poorest fit, thus optimizing phase separation. Surprisingly, shorter linkers mediate a transition to a gas of rods as Rubisco stickers approach the poles. These findings illustrate how intrinsically disordered proteins affect phase separation through the interplay of molecular length scales.