Chloroplast SRP43 acts as a chaperone for glutamyl-tRNA reductase, the rate-limiting enzyme in tetrapyrrole biosynthesis

Abstract

Assembly of light-harvesting complexes requires synchronization of chlorophyll (Chl) biosynthesis with biogenesis of light-harvesting Chl a/b-binding proteins (LHCPs). The chloroplast signal recognition particle (cpSRP) pathway is responsible for transport of nucleus-encoded LHCPs in the stroma of the plastid and their integration into the thylakoid membranes. Correct folding and assembly of LHCPs require the incorporation of Chls, whose biosynthesis must therefore be precisely coordinated with membrane insertion of LHCPs. How the spatiotemporal coordination between the cpSRP machinery and Chl biosynthesis is achieved is poorly understood. In this work, we demonstrate a direct interaction between cpSRP43, the chaperone that mediates LHCP targeting and insertion, and glutamyl-tRNA reductase (GluTR), a rate-limiting enzyme in tetrapyrrole biosynthesis. Concurrent deficiency for cpSRP43 and the GluTR-binding protein (GBP) additively reduces GluTR levels, indicating that cpSRP43 and GBP act nonredundantly to stabilize GluTR. The substrate-binding domain of cpSRP43 binds to the N-terminal region of GluTR, which harbors aggregation-prone motifs, and the chaperone activity of cpSRP43 efficiently prevents aggregation of these regions. Our work thus reveals a function of cpSRP43 in Chl biosynthesis and suggests a striking mechanism for posttranslational coordination of LHCP insertion with Chl biosynthesis.

Chloroplast SRP43 subunit Prevents Aggregation of Proteins

“…This theory is upheld in another recent study in which cpSRP43 has been identified as a chaperone for glutamyl-tRNA reductase (GluTR), the rate-limiting enzyme in tetrapyrrole biogenesis (49). Tetrapyrrole is at the core of chlorophyll and the biogenesis of chlorophyll is coordinated with that of LHCPs in order for the proper assembly of lightharvesting complexes to proceed (49). Here they found that a deficit of cpSRP43 as well as…”

“…GluTR-binding protein (GBP) significantly reduced production of GluTR and lead to a reduction in chlorophyll biogenesis which suggests that both cpSRP43 and GBP play distinct roles in the stabilization of GluTR (35). This group was able to form a ternary complex between cpSRP43, L18 and GluTR allowing them to speculate that both substrates are not competing for the same binding surface on cpSRP43 (49). Upon further examination, cpSRP43 was found to interact with the N-terminal of GluTR (49).…”

“…The binding interface proposed by McAvoy et al is primarily made up of hydrophobic residues suggesting that the protection mode of cpSRP43 is driven by the formation of hydrophobic interactions. This theory is upheld in another recent study in which cpSRP43 has been identified as a chaperone for glutamyl-tRNA reductase (GluTR), the rate-limiting enzyme in tetrapyrrole biogenesis (49). Tetrapyrrole is at the core of chlorophyll and the biogenesis of chlorophyll is coordinated with that of LHCPs in order for the proper assembly of lightharvesting complexes to proceed (49).…”

“…This group was able to form a ternary complex between cpSRP43, L18 and GluTR allowing them to speculate that both substrates are not competing for the same binding surface on cpSRP43 (49). Upon further examination, cpSRP43 was found to interact with the N-terminal of GluTR (49). The N-and C-terminal domains of GluTR can bind to different regulators to issue various control mechanisms for the enzyme (50) GBP was already known to interact with a heme-binding domain on the N-terminal of GluTR and this group conducted bimolecular fluorescence complementation assays (BiFC) and HIS pull-down assays in order to determine which GluTR domain interacts with cpSRP43 (49).…”

“…Upon further examination, cpSRP43 was found to interact with the N-terminal of GluTR (49). The N-and C-terminal domains of GluTR can bind to different regulators to issue various control mechanisms for the enzyme (50) GBP was already known to interact with a heme-binding domain on the N-terminal of GluTR and this group conducted bimolecular fluorescence complementation assays (BiFC) and HIS pull-down assays in order to determine which GluTR domain interacts with cpSRP43 (49). cpSRP43 was found to bind to GluTR-N64-163 which contains the heme-binding domain as well as part of a catalytic domain, adjacent to the heme-binding domain, in the N-terminal (49).…”