Shape of RBC is biconcave due to?

The most impoant constituent of the cytoskeleton is protein spectrin. Includes alpha and beta chains.Interwined spectrin molecules are linked together by other cytoskeletal proteins and actin to form a lattice like network which is attached to the internal surface of the membrane.This network is a resilent structure which normally causes red cells to resume the biconcave disc form after forces causing distoion are removed Ref:deGruchy's clinical haematology in medical practice 6th adapted edition.page no.16 Ref Robbins 9/e pg 410