16.16: Protein Transport to the Stroma
Chloroplasts are triple membrane structures with an outer membrane, an inner membrane, and a thylakoid membrane, each containing distinct metabolite transporters, membrane translocons, and enzymes. Appropriate sorting and translocating these proteins to their correct membrane systems is essential for chloroplast function.
Protein complexes called the translocon of the outer chloroplast membrane or TOC complex, and the translocon of the inner chloroplast membrane or TIC complex mediate the correct sorting of chloroplast proteins. Newly synthesized precursors targeted to the stroma bind cytosolic chaperones Hsp90 and Hsp70-Hsp90 Organizing Protein (HOP). Hsp90 and HOP use energy from ATP hydrolysis to keep the precursor in an unfolded state. A cleavable 13 to 146 amino acids long transit peptide sequence at the N-terminal and a non-cleavable internal signal sequence help precursors translocate to the correct chloroplast subcompartment. The transit peptide is recognized by two homologous GTPases, TOC159 and TOC34. TOC159 is the initial receptor for the transit peptide, while TOC34 facilitates GTP hydrolysis to help the precursor enter through the TOC channel.
The precursor is then transported across the inner membrane through the TIC complex. Sufficient ATP concentration coupled with proper temperature facilitates the translocation of the precursor across the outer and inner chloroplast membrane into the stroma. TIC40 also stimulates Hsp93 to utilize the energy of ATP hydrolysis and pull the precursor completely into the stroma. Stromal processing peptidases cleave the transit signal as the processed precursor is released into the stroma and folded by Cpn60 to form the active protein. Hsp93 undergoes ADP to ATP exchange and helps in consecutive cycles of import.