LTD coordinates chlorophyll biosynthesis and LIGHT-HARVESTING CHLOROPHYLL A/B-BINDING PROTEIN transport

Chlorophyll biosynthesis must be tightly coupled to light-harvesting chlorophyll a/b-binding protein (LHCP) biogenesis, as free chlorophyll and its precursors are phototoxic. However, precisely how these 2 processes are coordinated in Arabidopsis (Arabidopsis thaliana) remains elusive. Our previous studies demonstrated the role of LHCP TRANSLOCATION DEFECT (LTD) in delivering LHCPs to the chloroplast via the signal recognition particle-dependent pathway. Here, we show that LTD interacts with and stabilizes the chlorophyll biosynthesis enzymes Mg-protoporphyrin methyltransferase and Mg-protoporphyrin monomethylester (MgPME) cyclase, maintaining their activity. We also demonstrate the direct binding of LTD to MgPME, and through crystal structure analysis, we show that the groove of the LTD dimer is critical for MgPME binding. Thus, we propose that LTD transfers MgPME from Mg-protoporphyrin methyltransferase to the MgPME cyclase. These results elucidate a role for LTD in synchronizing chlorophyll biosynthesis with LHCP transport to ensure the correct insertion of chlorophylls into LHCPs.