Figure 2.

Stepwise formation of the pre-B cell receptor. Isolated λ₅ protein has an improperly folded structure in which Cλ₅ (◊, blue) has not yet attained an Ig-domain structure (○) and is associated with the amino terminal unique portion of λ₅, which acts as an intramolecular chaperone (). The β₇ strand of the amino terminal λ₅ portion is folded in an Ig-domain- like fashion (). The V_preB protein is folded as an Ig-like domain, missing the β₇-strand with a unique, non-Ig carboxyl terminal portion (, red). The isolated λ₅-protein cannot displace BIP and associate with the C_H1-domain of the μH chain, because neither λ₅ (◊) nor C_H1 (□, green) are properly Ig-domain-folded. Association of V_preB with λ₅ induces an Ig-domain-structure in Cλ₅ and displaces the intramolecular chaperone (). V_preB interacts with V_H of the μH chain, Cλ₅ displaces BIP and induces an Ig-domain structure in C_H1 (○, green), forms a disulfide bridge and thus, the pre-B cell receptor.