Oligomerization of the Tetraspanin CD81 via the Flexibility of Its δ-Loop.

Tetraspanins are master organizers in the plasma membrane, forming tetraspanin-enriched microdomains with one another and other surface molecules. Their rod-shaped structure includes a large extracellular loop (LEL) that plays a pivotal role in tetraspanin network formation. We performed comparative atomistic and coarse-grain molecular-dynamics simulations of the LEL in isolation and full-length CD81, and reproduced LEL flexibility patterns known from wet-lab experiments in which the LEL δ-loop region showed a pronounced flexibility. In a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine lipid bilayer and a plasma membrane environment, the conformational flexibility of the δ-loop initiates CD81-CD81 contacts for oligomerization. Furthermore, in the plasma membrane, CD81-ganglioside bridges arising from preformed glycolipid patches cross-link the complexes. The data suggest that exposing a flexible domain enables binding to interaction partners by circumventing the restriction of orientation and conformational freedom of membrane proteins.