Tentative model for a CCB-mediated apo- to holo-cyt b₆ conversion. Model with the following steps: membrane integration of apocyt b₆, formation of a b_L-heme–dependent intermediate that can be prevented by heme depletion (gabaculine treatment), and formation of a b_L and b_H binding cyt b₆ (Kuras et al., 1997); formation of a transient oligomer comprising CCB1/b₆, formation of a transient oligomer comprising CCB3/b₆, association of the oligomer comprising CCB3/b₆ with the stable heterodimer CCB2/CCB4 to form the heme ligation complex, and a transient oligomer comprising CCB2/4/3/b₆, which is stabilized in petB-C35V strain; c_i binding leads to a holocyt b₆ with three hemes; and cyt b₆ associates with other b₆f subunits even in the absence of c_i-heme covalent binding. The binding site of α-b₆N on cyt b₆ (N) is masked whenever this cyt interacts with CCB1 or CCB2/CCB4/CCB3, thereby preventing α-b₆N access to its epitope. The question mark indicates that CCB3/b₆ is a transient protein complex that may arise before formation or after dissociation of the CCB2/CCB4/CCB3/b₆ complex.