Practice question

Thylakoid membrane harbors photosynthetic machinery requiring insertion light-harvesting polytopic proteins after chloroplast envelope import stroma. Four conserved pathways inherited cyanobacterial ancestor: Sec pathway unfolded proteins SecYEG translocase SecA ATPase, SRP pathway specialized highly hydrophobic light-harvesting chlorophyll a/b binding proteins LHCPs pigments. Stromal chloroplast SRP cpSRP54 GTPase homologous bacterial SRP54 NG M domains binding signal sequence GTP, unique cpSRP43 chaperone absent bacteria three chromodomains four ankyrin repeats specifically binding L18 motif 18 amino acids between transmembrane helices two three LHCP preventing aggregation maintaining soluble transit competent essential pigments otherwise aggregate uncontrollably. Cargo-cpSRP complex targets thylakoid membrane via cpFtsY GTPase homologous FtsY SRalpha then hands substrate Alb3 insertase Oxa1 YidC family integrating bilayer lateral gate GTP hydrolysis dependent. OXA pathway mitochondria inserts inner proteins matrix side TIM23 handles presequence translocation mitochondrial inner membrane MIA40 mediates oxidative disulfide mitochondrial IMS none relevant thylakoid. Loss cpSRP43 Arabidopsis chaos pale green phenotype severe loss LHCPs demonstrating indispensable role thylakoid assembly functional chloroplast development and photosynthesis efficiency and light harvesting.