Abstract
The universally conserved signal recognition particle (SRP) co-translationally delivers newly synthesized membrane and secretory proteins to the target cellular membrane. The only exception is found in the chloroplast of green plants, where the chloroplast SRP (cpSRP) post-translationally targets light-harvesting chlorophyll a/b-binding proteins (LHCP) to the thylakoid membrane. The mechanism and regulation of this post-translational mode of targeting by cpSRP remain unclear. Using biochemical and biophysical methods, here we show that anionic phospholipids activate the cpSRP receptor cpFtsY to promote rapid and stable cpSRP54·cpFtsY complex assembly. Furthermore, the stromal domain of the Alb3 translocase binds with high affinity to and regulates GTP hydrolysis in the cpSRP54·cpFtsY complex, suggesting that cpFtsY is primarily responsible for initial recruitment of the targeting complex to Alb3. These results suggest a new model for the sequential recruitment, remodeling, and unloading of the targeting complex at membrane translocase sites in the post-translational cpSRP pathway.
Highlights
Localization of proteins to their correct cellular destinations is essential for proper cellular function
Alb3-CTD on GTPase activation was observed with and without cpSRP43 present (Figs. 4A and 5C). These results demonstrate that the regulatory effect of Alb3 is specific and only occurs in a cpSRP541⁄7cpFtsY GTPase complex formed in the presence of anionic phospholipids and in which the cpSRP54 M-domain is properly positioned near the FtsY
Accurate timing of complex assembly, GTPase activation, and hydrolysis of the signal recognition particle (SRP) and SRP receptor (SR) GTPases is crucial for efficient protein targeting
Summary
The universally conserved signal recognition particle (SRP) co-translationally delivers newly synthesized membrane and secretory proteins to the target cellular membrane. The stromal domain of the Alb translocase binds with high affinity to and regulates GTP hydrolysis in the cpSRP541⁄7cpFtsY complex, suggesting that cpFtsY is primarily responsible for initial recruitment of the targeting complex to Alb. Interaction of PG and cardiolipin with this motif stimulates complex assembly between bacterial SRP and SR by pre-organizing SR into a more active conformation [41] This lipid-binding motif is conserved in cpFtsY and is necessary for targeting LHCP to the thylakoid membrane [42]. Alb3-CTD binds with submicromolar affinity to the cpSRP541⁄7cpFtsY complex and regulates GTP hydrolysis in this complex These results demonstrate extensive spatial regulation of the SRP and SR GTPases in the cpSRP system and suggest a new model for the recruitment of the targeting complex to the membrane translocase during LHCP targeting
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