Bovine binder-of-sperm protein BSP1 promotes protrusion and nanotube formation from liposomes

Binder-of-sperm (BSP) proteins interact with sperm membranes and are proposed to extract selectively phosphatidylcholine and cholesterol from these. This change in lipid composition is a key step in sperm capacitation. The present work demonstrates that the interactions between the protein BSP1 and model membranes composed with phosphatidylcholine lead to drastic changes in the morphology of the lipidic self-assemblies. Using cryo-electron microscopy and fluorescence microscopy, we show that, in the presence of the protein, the lipid vesicles elongate, and form bead necklace-like structures that evolve toward small vesicles or thread-like structures. In the presence of multilamellar vesicles, where a large reservoir of lipid is available, the presence of BSP proteins lead to the formation of long nanotubes. Long spiral-like threads, associated with lipid/protein complexes, are also observed. The local curvature of lipid membranes induced by the BSP proteins may be involved in lipid domain formation and the extraction of some lipids during the sperm maturation process.

Research highlights
► Binder-of-sperm protein 1 (BSP1) modifies the morphology of lipidic vesicles inducing bead necklace-like and thread-like structures.
► In the presence of multilamellar liposomes, BSP1 leads to the formation of long nanotubes.
► The insertion of BSP1 in the external lipid leaflet of membranes induces local changes in bilayer curvature.