(A) Assembly of MC-HSP90 hexameric structure. The process begins with a monomer (orange) subunit associating with another monomer (green) to form a stable dimer as the building block. It continues with further association of dimers via the N-terminal interface of M domain to form a possible dimer of dimers (or tetramer) and a stable hexamer. The occurrence of tetramer might represent an intermediate. In the models of MC-HSP90 dimer, tetramer and hexamer shown here, the building blocks are paired and indicated by the labels AA′, BB′ and CC′. (B) Top view and side view of HSP90N hexamer with 12 flexible polypeptide segments hanging around the core structure. The protomers of hexameric HSP90N are represented by different colors, green, orange, blue, yellow, red, and megenta. The N-terminal hydrophilic extension and C-terminal tail of the yellow protomer are labeled.