1.A.18 The Chloroplast Envelope Anion Channel-forming Tic110 (Tic110) Family
The Tic110 protein of the inner chloroplast envelope membrane is a constituent of the inner membrane protein import apparatus. It has been designated (1) Tic110, (2) IEP110, (3) IAP100 and (4) protein import- related anion channel (PIRAC) by various investigators. It is 996 residues long and exhibits 2 putative TMSs near its N-terminus at positions 74-92 and 101-120. Biochemical analyses suggest that this protein is part of a 600 kDa complex (generated by cross linking an in transit precursor protein to the translocation complex). Most of the Tic110 protein is probably in the intermembrane space (between the outer and inner membranes) (Lübeck et al., 1996). In this system, transport across the outer and inner membranes probably occurs by two independent processes (Kessler and Blobel, 1996).
Tic110 appears to be the protein import-related anion-selective channel. The evidence includes complete inactivation of the channel activity by addition of anti Tic110 antibody. The pore size was estimated to be about 6.5Å (van den Wijngaard and Vredenberg, 1999). Tic110 has two proposed functions with naturally exclusive structures; a protein-conducting channel with 6 TMSs, and a scaffold with 2 N-terminal TMSs followed by a large soluble domain for binding transit peptides and other stromal translocon components. The x-ray structure (4.2Å resolution) of Tic110B and C from Cyanidioschyzon merolae is known (Tsai et al. 2013). The C-terminal half of Tic110 posesses 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 (Tsai et al. 2013).
The transport reactions across the chloroplast inner membrane catalyzed by Tic110 are:
(1) anions ⇌ anions
(2) proteins (out) ⇌ proteins (in)