An unexpected role for mitochondrial ClpX

The AAA+ unfoldase ClpX contributes to bacterial proteostasis. In eukaryotes ClpX is found only in mitochondria (mtClpX), and its function has so far remained elusive. Baker and colleagues now report that mtClpX plays a part in heme biosynthesis by facilitating synthesis of the precursor 5-aminolevulinic acid (ALA). Searching large-scale genetic- and chemical-interaction data in Saccharomyces cerevisiae, they found that the interaction profile of the gene encoding the yeast mtClpX homolog Mcx1 is strongly correlated with those of the genes involved in the initial steps of heme biosynthesis, including HEM1, which encodes the ALA synthase Hem1. Deletion or function-impairing mutation of Mcx1 reduces cellular heme levels, but the phenotype is rescued by supplementation with ALA, thus suggesting a direct role of Mcx1 in ALA synthesis. ALA synthases, including Hem1, require the cofactor pyridoxal phosphate (PLP) for enzymatic activity. Baker and colleagues found that Mcx1 directly interacts with Hem1 and increases PLP binding to the apo-Hem1 by eight-fold, thus facilitating its activation. Although PLP binding requires Hem1 to maintain at least a partially folded state, Hem1 activation is nevertheless dependent on the unfoldase function of Mcx1. These observations led the authors to propose that upon interacting with the apoenzyme, mtClpX may locally unfold ALA synthase, thus exposing its buried active site to facilitate PLP binding. Consistently with the high sequence conservation of mtClpX and ALA synthase in yeast and mammals, mouse mtClpX also stimulates PLP activation of a human ALA synthase apoenzyme in vitro; this suggests that the mechanism identified in yeast is conserved across eukaryotes. Finally, Baker and colleagues found that knockdown of mtClpX in zebrafish embryos impairs erythropoiesis, consistently with the high requirement for heme to form hemoglobin during this process. These findings shed light on the role of ClpX in mitochondria and indicate an unexpected function for AAA+ unfoldases. (Cell 161, 858–867, 2015)