Liquid-Liquid Phase Transition Drives Intra-chloroplast Cargo Sorting

• Published in: Cell Vol. 180; no. 6; pp. 1144 - 1159.e20
• Main Authors: Ouyang, Min, Li, Xiaoyi, Zhang, Jing, Feng, Peiqiang, Pu, Hua, Kong, Lingxi, Bai, Zechen, Rong, Liwei, Xu, Xiumei, Chi, Wei, Wang, Qiang, Chen, Fan, Lu, Congming, Shen, Jianren, Zhang, Lixin
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 19.03.2020
• Subjects: ankyrin-repeat proteins; cargo sorting; chloroplast; intrinsically disordered regions; liquid-liquid phase separation; nuclear-encoded chloroplast proteins; thylakoid membranes; twin arginine translocation pathway; twin arginine translocation pathway; ankyrin-repeat proteins; cargo sorting; chloroplast; liquid-liquid phase separation; thylakoid membranes; nuclear-encoded chloroplast proteins; intrinsically disordered regions
• ISSN: 0092-8674; 1097-4172
• DOI: 10.1016/j.cell.2020.02.045

In eukaryotic cells, organelle biogenesis is pivotal for cellular function and cell survival. Chloroplasts are unique organelles with a complex internal membrane network. The mechanisms of the migration of imported nuclear-encoded chloroplast proteins across the crowded stroma to thylakoid membranes are less understood. Here, we identified two Arabidopsis ankyrin-repeat proteins, STT1 and STT2, that specifically mediate sorting of chloroplast twin arginine translocation (cpTat) pathway proteins to thylakoid membranes. STT1 and STT2 form a unique hetero-dimer through interaction of their C-terminal ankyrin domains. Binding of cpTat substrate by N-terminal intrinsically disordered regions of STT complex induces liquid-liquid phase separation. The multivalent nature of STT oligomer is critical for phase separation. STT-Hcf106 interactions reverse phase separation and facilitate cargo targeting and translocation across thylakoid membranes. Thus, the formation of phase-separated droplets emerges as a novel mechanism of intra-chloroplast cargo sorting. Our findings highlight a conserved mechanism of phase separation in regulating organelle biogenesis. [Display omitted] •A complex of STTs acts in sorting chloroplast Tat substrates to the thylakoids•Tat substrate activates the STT complex to undergo phase separation•Hcf106 reverses phase separation and facilitates cargo translocation A pair of ankyrin repeat proteins in chloroplasts recognizes signal peptides on thylakoid-bound cargo proteins and undergoes liquid-liquid phase separation to facilitate their sorting and transport.