Figure 7.

A Model for the Role of GluTRBP in the Spatial Separation of GluTR and ALA Synthesis for Tetrapyrrole Biosynthesis in Chloroplasts.

It is suggested that GluTRBP contributes to the spatial separation of ALA synthesis for heme and chlorophyll synthesis. GluTRBP anchors a small part of the total amount of GluTR at the thylakoid membrane. ALA formed in this subcompartment is dedicated to heme biosynthesis, when in parallel the majority of GluTR is located in the stroma of plastids to provide ALA for chlorophyll biosynthesis. A direct physical interaction of POR and FLU has not been demonstrated yet. However, in response to accumulating protochlorophyllide, the dominant part of ALA biosynthesis is inactivated when GluTR interacts with FLU. The FLU-dependent inactivation is proposed not to interfere with the GluTR-GluTRBP portion belonging to the separated ALA synthesis. Thus, heme synthesis can independently continue. Consequently, heme may act as feedback inhibitor on ALA biosynthesis through transcriptional or posttranslational control.