Structural characterizations of the chloroplast translocon protein Tic110

Tic110 is a major component of the chloroplast protein import translocon. Two functions with mutually exclusive structures have been proposed for Tic110: a protein‐conducting channel with six transmembrane domains and a scaffold with two N‐terminal transmembrane domains followed by a large soluble domain for binding transit peptides and other stromal translocon components. To investigate the structure of Tic110, Tic110 from Cyanidioschyzon merolae (CmTic110) was characterized. We constructed three fragments, CmTic110_A, CmTic110_B and CmTic110_C, with increasing N‐terminal truncations, to perform small‐angle X‐ray scattering (SAXS) and X‐ray crystallography analyses and Dali structural comparison. Here we report the molecular envelope of CmTic110_B and CmTic110_C determined by SAXS, and the crystal structure of CmTic110_C at 4.2 Å. Our data indicate that the C‐terminal half of CmTic110 possesses a rod‐shaped helix‐repeat structure that is too flattened and elongated to be a channel. The structure is most similar to the HEAT‐repeat motif that functions as scaffolds for protein–protein interactions.