A specific group of transmembrane receptors, including the β1-adrenergic receptor (β1-AR), is internalized through a non-clathrin pathway known as Fast Endophilin Mediated Endocytosis (FEME). A key question is: how does the endocytic machinery assemble and how is it modulated by activated receptors during FEME. Here we show that endophilin, a major regulator of FEME, undergoes a phase transition into liquid-like condensates, which facilitates the formation of multi-protein assemblies by enabling the phase partitioning of endophilin binding proteins. The phase transition can be triggered by specific multivalent binding partners of endophilin in the FEME pathway such as the third intracellular loop (TIL) of the β1-AR, and the C-terminal domain of lamellipodin (LPD). Other endocytic accessory proteins can either partition into, or target interfacial regions of, these condensate droplets, and LPD also phase separates with the actin polymerase VASP. On the membrane, TIL promotes protein clustering in the presence of endophilin and LPD C-terminal domain. Our results demonstrate how the multivalent interactions between endophilin, LPD, and TIL regulate protein assembly formation on the membrane, providing mechanistic insights into the priming and initiation steps of FEME.

Metal-ion-mediated interactions between calcium-binding peripheral proteins and membrane phospholipids is a key feature of multiple cell signaling processes. The molecular basis for the interaction involves the displacement of inner-sphere water molecules on calcium ions by phosphate groups of the phospholipids. On the basis of this fundamental mechanism, we have devised a novel "turn-on" optical sensing strategy for anionic phospholipids by using a lanthanide reconstituted protein. The "lanthano" protein turns on selectively in the presence of a crucial signaling phospholipid, phosphatidylserine, by affording a 6 times enhancement in lanthanide luminescence. The "turn-on" sensing strategy was distinctly validated by direct evidence for the water-displacement mechanism via lifetime measurements.

Phosphoinositides are critical cell-signal mediators present on the plasma membrane. The dynamic change of phosphoinositide concentrations on the membrane including clustering and declustering mediates signal transduction. The importance of phosphoinositides is scored by the fact that they participate in almost all cell-signaling events, and a defect in phosphoinositide metabolism is linked to multiple diseases including cancer, bipolar disorder, and type-2 diabetes. Optical sensors for visualizing phosphoinositide distribution can provide information on phosphoinositide dynamics. This exercise will ultimately afford a handle into understanding and manipulating cell-signaling processes. The major requirement in phosphoinositide sensor development is a selective, cell permeable probe that can quantify phosphoinositides. To address this requirement, we have developed short peptide-based ratiometric fluorescent sensors for imaging phosphoinositides. The sensors afford a selective response toward two crucial signaling phosphoinositides, phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) and phosphatidylinositol-4-phosphate (PI4P), over other anionic membrane phospholipids and soluble inositol phosphates. Dissociation constant values indicate up to 4 times higher probe affinity toward PI(4,5)P2 when compared to PI4P. Significantly, the sensors are readily cell-permeable and enter cells within 15 min of incubation as indicated by multiphoton excitation confocal microscopy. Furthermore, the sensors light up signaling phosphoinositides present both on the cell membrane and on organelle membranes near the perinuclear space, opening avenues for quantifying and monitoring phosphoinositide signaling.

Endophilin plays key roles during endocytosis of cellular receptors, including generating membrane curvature to drive internalization. Electrostatic interactions between endophilin’s BAR domain and anionic membrane lipids have been considered the major driving force in curvature generation. However, the SH3 domain of endophilin also interacts with the proline-rich third intracellular loop (TIL) of various G-protein coupled receptors (GPCRs), and it is unclear whether this interaction has a direct role in generating membrane curvature during endocytosis. To examine this, we designed model membranes with a membrane density of 1400 receptors per µm2 represented by a covalently conjugated TIL region from β1-adrenergic receptor. We observed that TIL recruits endophilin to membranes composed of 95 mol% of zwitterionic lipids via the SH3 domain. More importantly, endophilin recruited via TIL tubulates vesicles and gets sorted onto highly curved membrane tubules. These observations indicate that the cellular membrane bending and curvature sensing activities of endophilin can be facilitated through detection of the TIL of activated GPCRs in addition to binding to anionic lipids. Furthermore, we show that TIL electrostatically interacts with membranes composed of anionic lipids. Therefore, anionic lipids can modulate TIL/SH3 domain binding. Overall, our findings imply that an interplay between TIL, charged membrane lipids, BAR domain, and SH3 domain could exist in the biological system and that these components may act in coordination to regulate the internalization of cellular receptors.

We present a systematic experimental and computational study of phospholipid induced peptide coil-helix transitions which are relevant in the context of proteins mediating cytoskeletal rearrangement via membrane binding. We developed a sensitive Förster resonance energy transfer (FRET) based assay to address whether coil-helix transitions in phospholipid binding motifs of actin-binding proteins can be induced by physiologically-relevant concentrations (1-20 μM) of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) phospholipids. Based on inter-residue distance constraints obtained from Molecular Dynamics (MD) simulations of a 20 residue peptide (Gel 150-169) from the actin-severing protein gelsolin, we synthetized and labeled the peptide with a tryptophan donor and IAEDANS acceptor pair. Upon addition of PI(4,5)P2 micelles and mixed vesicles containing PI(4,5)P2 and phosphatidylcholine to the peptide, we observed a decrease in the tryptophan emission intensity with increasing concentrations of PI(4,5)P2. The IAEDANS emission spectra showed a more complex profile exhibiting a blue shift of the emission peak and non-monotonic changes in the intensity profile with increasing concentrations of PI(4,5)P2. We showed that the IAEDANS acceptor emission response is a result of both intrinsic polarity sensitivity of the acceptor in the vicinity of the membrane surface and fluorescence energy transfer from the donor. Importantly, the fluorescence lifetime of the donor (tryptophan) showed a monotonous decrease with increasing mol% of PI(4,5)P2 from 1.13 ± 0.10 ns in the absence of phospholipids to 0.25 ± 0.03 ns in the presence of 100% PI(4,5)P2 micelles. We also showed a concomitant increase in FRET efficiency with increasing PI(4,5)P2 levels indicating a PI(4,5)P2 concentration dependent coil-helix transition. Our studies demonstrate that membrane PI(4,5)P2 concentrations as low as 2.5-5 μM can trigger helix-coil conformational changes in gelsolin relevant for triggering regulatory processes in the cell.