Complex structure reveals CcmM and CcmN form a heterotrimeric adaptor in β-carboxysome.

Carboxysome is an icosahedral self-assembled microcompartment that sequesters RuBisCO and carbonic anhydrases within a selectively permeable protein shell. The scaffolding proteins, CcmM, and CcmN were proposed to act as adaptors that crosslink the enzymatic core to shell facets. However, the details of interaction pattern remain unknown. Here we obtained a stable heterotrimeric complex of CcmM γ-carbonic anhydrase domain (termed CcmM ) and CcmN, with a 1:2 stoichiometry, which interacts with the shell proteins CcmO and CcmL in vitro. The 2.9 Å crystal structure of this heterotrimer revealed an asymmetric bundle composed of one CcmM and two CcmN subunits, all of which adopt a triangular left-handed β-helical barrel structure. The central CcmN subunit packs against CcmM and another CcmN subunit via a wall-to-edge or wall-to-wall pattern, respectively. Together with previous findings, we propose CcmM -CcmN functions as an adaptor to facilitate the recruitment of shell proteins and the assembly of intact β-carboxysome.