FC2 stabilizes POR and suppresses ALA formation in the tetrapyrrole biosynthesis pathway

Abstract

During photoperiodic growth, the light-dependent nature of chlorophyll synthesis in angiosperms necessitates robust control of the production of 5-aminolevulinic acid (ALA), the rate-limiting step in the initial stage of tetrapyrrole biosynthesis (TBS). We are interested in dissecting the post-translational control of this process, which suppresses ALA synthesis for chlorophyll synthesis in dark-grown plants. Using biochemical approaches for analysis of wild-type and mutant lines as well as complementation lines, we show that the heme-synthesizing ferrochelatase 2 (FC2) interacts with protochlorophyllide oxidoreductase and the regulator FLU which both promote the feedback-controlled suppression of ALA synthesis by inactivation of glutamyl-tRNA reductase, thus preventing excessive accumulation of potentially deleterious tetrapyrrole intermediates. Thereby FC2 stabilizes POR by physical interaction. When the interaction between FC2 and POR is perturbed, suppression of ALA synthesis is attenuated and photoreactive protochlorophyllide accumulates. FC2 is anchored in the thylakoid membrane via its membrane-spanning CAB (chlorophyll-a-binding) domain. FC2 is one of the two isoforms of ferrochelatase catalyzing the last step of heme synthesis. Although FC2 belongs to the heme-synthesizing branch of TBS, its interaction with POR potentiates the effects of the GluTR-inactivation complex on the chlorophyll-synthesizing branch, and ensures reciprocal control of chlorophyll and heme synthesis.

Retrograde signaling in plants: A critical review focusing on the GUN pathway and beyond

“…The previous allocation of FC-mediated heme synthesis to plant mitochondria might be explained by technical limitations in purification of these organelles. Fan et al. (2022) present additional evidence showing that the two Arabidopsis FC isoforms are localized in different sub-compartments of chloroplasts.…”

“…However, more recent studies suggested that A. thaliana FC is exclusively located within the plastids ( Fan et al., 2022 ). The previous allocation of FC-mediated heme synthesis to plant mitochondria might be explained by technical limitations in purification of these organelles.…”

“…FC2, the dominant form, is localized in the thylakoid membrane and serves the need for heme-dependent proteins in plastids. FC1 is found in the stroma and, in lower amounts, in the thylakoids and, importantly, envelope membranes ( Fan et al., 2022 ). Moreover, it is known that FC1 is expressed at higher levels in non-photosynthetic tissues and that FC1 is induced in response to multiple abiotic stresses, when chloroplast functions are basically compromised ( Singh et al., 2002 ; Nagai et al., 2007 ; Scharfenberg et al., 2015 ; Zhao et al., 2017 ).…”