Caveolin in Bilayers: Can the Intramembrane U-Shaped Conformation Really Exist

Abstract

Caveolin induces membrane curvature and drives the formation of caveolae that participate in many crucial cell functions such as endocytosis. The central portion of caveolin-1 contains two helices (H1 and H2) connected by a three-residue break with both N- and C-termini exposed to the cytoplasm. Although a U-shaped configuration is assumed based on its inaccessibility by extracellular matrix probes, caveolin structure in a bilayer remains elusive. This work aims to characterize the structure and dynamics of caveolin-1 (D82 to S136; Cav182-136) in a DMPC bilayer using NMR, fluorescence emission measurements, and molecular dynamics (MD) simulations. The secondary structure of Cav182-136 from NMR chemical shift indexing analysis serves as guideline for generating initial structural models. 50 independent MD simulations (80 ns each) are performed to identify its favorable conformation and orientation in the bilayer. Using the short simulations as a guideline, a representative configuration was chosen and simulated for 1 μs to further explore its stability and dynamics. The results of these simulations mirror those from the tryptophan fluorescence measurements (i.e., Cav182-136 insertion depth in the bilayer), and corroborate that Cav182-136 inserts in the membrane with a U-shaped conformation. The angle between H1 and H2 ranges from 35° to 69°, and the tilt angle of Cav182-136 is 27° ± 6°. The simulations also show that specific faces of H1 and H2 prefer to interact with each other and with lipid molecules, and these interactions stabilize the U-shaped conformation.