FBAR syndapin 1 recognizes and stabilizes highly curved tubular membranes in a concentration dependent manner

Syndapin 1 FBAR, a member of the Bin-amphiphysin-Rvs (BAR) domain protein family, is known to induce membrane curvature and is an essential component in biological processes like endocytosis and formation and growth of neurites. We quantify the curvature sensing of FBAR on reconstituted porcine brain lipid vesicles and show that it senses membrane curvature at low density whereas it induces and reinforces tube stiffness at higher density. FBAR strongly up-concentrates on the high curvature tubes pulled out of Giant Unilamellar lipid Vesicles (GUVs), this sorting behavior is strongly amplified at low protein densities. Interestingly, FBAR from syndapin 1 has a large affinity for tubular membranes with curvatures larger than its own intrinsic concave curvature. Finally, we studied the effect of FBAR on membrane relaxation kinetics with high temporal resolution and found that the protein increases relaxation time of the tube holding force in a density-dependent fashion

Sensing and Stiffening of Tubular Membranes by the Syndapin 1 FBAR

Bin-Amphiphysin-Rvs (BAR) domains are essential components of the cellular machinery responsible for membrane deformation and were found to be sensitive sensors of membrane curvature. One common feature of BAR domains is a curved shape which correlates with high membrane curvatures often found in cells. We focused on characterizing the FBAR domain of Syndapin-1 in vitro, given its medical relevance to neurological diseases, using tubular membranes pulled out of Giant Unilamellar Vesicles (GUVs). Specifically, GUVs were synthesized using either porcine brain lipid extracts or the more conventional, binary lipid mixtures to systematically test how lipid composition affects curvature sensing activity. Using optical trapping coupled with force-spectroscopy and confocal microscopy, we discovered an inverse relationship between the curvature sensing activity of the FBAR domain and its equilibrium concentration in solution. At high bulk concentrations of protein, we explicitly measured an increase in the tube's persistence length, which can be understood as mechanical stiffening of the tube. Lastly, we used force spectroscopy to accurately test the effect of the protein on membrane relaxation dynamics in real time