Import of proteins into the chloroplast stroma

2. Import of proteins into the chloroplast stroma Tic complex Tic22 was identified as a soluble protein that is assumed to be the first Tic component interacting with the incoming precursor protein. Tic20 is integral membrane protein presumably involved in the formation of the protein translocation pore.

3. Import of proteins into the chloroplast stroma • Tic110 is shown to form a cation selective channel(阳离子选择通道).Moreover it contains a large hydrophilic（亲水的） domain which extends into the stroma where it might provide the docking site for precursor proteins and molecular chaperones（分子伴侣）. • Tic55, Tic32, and Tic62, on the other hand, might be involved in the proposed redox regulation of chloroplast protein import .

4. protein transport into thylakoid lumen ☆protein destined for the thylakoid should contain both a stroma-targeting domain(基质靶结构域) and a thylakiod-transfer domain(类囊体转移结构域) at their N-terminus, which is removed by thylakoidal processing peptidase(TPP) after enter into the lumen. ☆many of the proteins which are encoded by chloroplast genes and synthesized on the membrane-bound ribosome are reside within thylakoid membrane.

5. protein transport into thylakoid lumen Distinct pathways: ★Sec-dependent pathway ★SRP-dependent pathway ★ ⊿pH/Tat-dependent pathway ★ ‘spontaneous’ pathway Tat:twin arginine translocation

6. protein transport into thylakoid lumen The SRP-dependent pathway is energized by hydrolysis of GTP and by the transthylakoidal proton gradient in an as yet unknown manner. The SRP-dependent transport is exclusively targeting integral membrane proteins and are specific for the insertion of proteins into the thylakoid membrane .

7. protein transport into thylakoid lumen • SRP-dependent transport provides the major pathway for the integration of polytopic thylakoid membrane proteins. The main representative of this class of thylakoid proteins —— LHCP which isthe apoprotein(脱辅基蛋白） of the light harvesting complex associated with photosystem II.

8. protein transport into thylakoid lumen Sec and ⊿pH/Tat-dependent pathways: They are responsible for the translocation of hydrophilic proteins into the thylakoid lumen. Typical substrates of the thylakoidal Sec-dependent pathway areplastocyanin(质体蓝素), the photosystem I subunit PsaF, and the 33 kDa protein of the oxygen evolving system. In contrast, proteins like the 16 and 23 kDa subunits of the oxygen evolving system are translocated by the ⊿pH/Tat-dependent pathway .

9. protein transport into thylakoid lumen • Protein transport by ⊿pH/Tat-dependent pathway is mediated by thylakoid targeting signal peptides which carry a twin pair of arginine residues（精氨酸残基）upstream of the hydrophobic core domain, in contrast to the single lysine residue which is generally found in Sec-targeting signal peptides at this position.

10. protein transport into thylakoid lumen • Tat pathway is to translocate fully folded proteins or protein domains across the thylakoid membrane which obviously has significant implications on both the transport mechanism as well as the structure and function of the translocase. • Three components of the thylakoidal ⊿pH/Tat transport machinery have been identified to date, called TatA (or Tha4), TatB (or Hcf106), and TatC (or cpTatC )

11. thanks