What is the main function of the TIC complex in chloroplasts?
Chloroplast envelope encloses two translocon systems tandem importing majority organelle proteome 3000 proteins despite plastid genome about 100. Precursors synthesized cytosol N-terminal transit peptide enriched hydroxylated serine threonine positively charged amphipathic not hydrophobic distinct mitochondrial presequence. First receptor TOC outer envelope: Toc159 large 159 kDa GTPase selectivity filter photosynthetic versus housekeeping via acidic A-domain, Toc34 small 34 kDa regulatory, Toc75 Omp85 beta-barrel superfamily 14-strand channel 3 nm pore translocating polypeptide. After outer membrane TOC hands substrate TIC inner envelope: Tic110 main cation-selective channel large intermembrane space domain recruiting peptide stromal scaffold binding chaperones, Tic40 co-chaperone Sti1-like TPR recruiting Hsp93 ClpC AAA ATPase ATP-driven motor pulling substrate, Tic20 alternative smaller channel, Tic22 soluble intermembrane bridging, Tic236 giant forming supercomplex spanning both membranes creating direct continuous contact sites. Upon arrival stroma transit peptide cleaved stromal processing peptidase SPP metalloenzyme producing mature protein. TIC does not import across outer envelope TOC function does not direct Golgi plastids lack connections nor recycle; exclusive role inner envelope passage stromal compartment preceding further thylakoid sorting Sec SRP Tat pathways essential development and photosynthesis.
Ref: Soll & Schleiff, Nat Rev Mol Cell Biol 2004: TIC imports proteins across chloroplast inner envelope.