Abstract
Proteins that are synthesized on cytoplasmic ribosomes but function within plastids must be imported and then targeted to one of six plastid locations. Although multiple systems that target proteins to the thylakoid membranes or thylakoid lumen have been identified, a system that can direct the integration of inner envelope membrane proteins from the stroma has not been previously described. Genetics and localization studies were used to show that plastids contain two different Sec systems with distinct functions. Loss-of-function mutations in components of the previously described thylakoid-localized Sec system, designated as SCY1 (At2g18710), SECA1 (At4g01800), and SECE1 (At4g14870) in Arabidopsis (Arabidopsis thaliana), result in albino seedlings and sucrose-dependent heterotrophic growth. Loss-of-function mutations in components of the second Sec system, designated as SCY2 (At2g31530) and SECA2 (At1g21650) in Arabidopsis, result in arrest at the globular stage and embryo lethality. Promoter-swap experiments provided evidence that SCY1 and SCY2 are functionally nonredundant and perform different roles in the cell. Finally, chloroplast import and fractionation assays and immunogold localization of SCY2-green fluorescent protein fusion proteins in root tissues indicated that SCY2 is part of an envelope-localized Sec system. Our data suggest that SCY2 and SECA2 function in Sec-mediated integration and translocation processes at the inner envelope membrane.
Highlights
Proteins that are synthesized on cytoplasmic ribosomes but function within plastids must be imported and targeted to one of six plastid locations
According to the conservative sorting hypothesis (Hartl et al, 1986), proteins that are destined for the inner envelope membrane, which corresponds to the plasma membrane of the original bacterial endosymbiont, should use systems and mechanisms related to those involved in secretion and membrane protein integration in bacteria
A second gene encoding a homolog of SecA, the ATPase driving translocation and integration events, was identified. We show that this protein (SECA2) is targeted to plastids and that mutants have the same loss-of-function phenotype as scy2 mutants. Because this translocation/integration system is associated with the envelope, we propose that it may be important for postimport targeting of inner membrane proteins and/or early stages of thylakoid biogenesis
Summary
Proteins that are synthesized on cytoplasmic ribosomes but function within plastids must be imported and targeted to one of six plastid locations. Loss-of-function mutations in components of the previously described thylakoid-localized Sec system, designated as SCY1 (At2g18710), SECA1 (At4g01800), and SECE1 (At4g14870) in Arabidopsis (Arabidopsis thaliana), result in albino seedlings and sucrose-dependent heterotrophic growth. Our data suggest that SCY2 and SECA2 function in Sec-mediated integration and translocation processes at the inner envelope membrane. Sec Translocases in Plastids require a postimport pathway for integration, a translocase that mediates insertion into the inner membrane or translocation to the intramembrane space has not been identified (Tripp et al, 2007). Four different pathways have been described for integration of membrane proteins or translocation to the thylakoid lumen. Indirect evidence suggests that cpSRP may function in conjunction with SecY to integrate plastid-encoded thylakoid proteins (Cline and Theg, 2007). The spontaneous pathway is not thought to involve proteinaceous components in the membrane
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
