An adaptor for feedback regulation of heme biosynthesis by a mitochondrial protease.
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| Title: | An adaptor for feedback regulation of heme biosynthesis by a mitochondrial protease. |
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| Authors: | Cottle, Thomas (AUTHOR), Joh, Lydia (AUTHOR), Posner, Cori (AUTHOR), DeCosta, Adam (AUTHOR), Campagna, Dean R. (AUTHOR), Fleming, Mark D. (AUTHOR), Ducamp, Sarah (AUTHOR), Kardon, Julia R. (AUTHOR) |
| Source: | Science. 7/30/2026, Vol. 393 Issue 6810, p1-9. 9p. |
| Abstract: | Heme biosynthesis is tightly coordinated to support essential functions without accumulating toxic porphyrins and depleting cellular iron. Heme induces degradation of the heme biosynthetic enzyme, 5-aminolevulinate synthase (ALAS), by the mitochondrial caseinolytic protease complex CLPX-CLPP (CLPXP), but the mechanism for heme-triggered degradation had not been elucidated. We found that polymerase delta-interacting protein 2 (POLDIP2) is a heme-sensing adaptor protein sufficient to reconstitute negative feedback degradation of ALAS by CLPXP. POLDIP2 was necessary to support ALAS turnover in cells and regulate heme production during erythropoiesis. POLDIP2 directly recognized and recruited heme-bound ALAS to CLPXP. Degradation initiation required a carboxyl-terminal element of ALAS, truncations of which cause an erythropoietic protoporphyria. Our findings establish a mechanism for conditional degradation by CLPXP that underlies erythropoietic protoporphyrias linked to CLPX and ALAS. Editor's summary: The molecule heme has important functions in oxygen transport, as a cofactor for enzymes, and as a sensor. It must be available as needed, but excessive synthesis can cause the accumulation of toxic precursors or depletion of iron, so its production is tightly controlled. The unfoldase protein CLPX activates 5-aminolevulinate synthase (ALAS), the rate-limiting enzyme for heme synthesis, but balances this activity as part of the proteolytic complex CLPXP, which degrades ALAS isozymes. Cottle et al. report that this switch requires polymerase delta-interacting protein 2 (POLDIP2). This protein binds heme-bound ALAS and acts as an adaptor that promotes CLPXP-dependent degradation of ALAS and may thus help to modulate heme production during erythropoiesis. —L. Bryan Ray INTRODUCTION: Biosynthesis of heme is tightly coordinated with cellular demand, such that vital functions of this essential cofactor, including oxygen transport, respiration, and catalysis, are fully supplied without accumulating toxic heme precursors or depleting cellular iron. Heme biosynthesis begins in the mitochondrial matrix with the rate-limiting enzyme 5-aminolevulinate synthase (ALAS). One mechanism through which ALAS activity is controlled is by negative feedback degradation triggered by a heme-binding site in ALAS. This degradation requires the mitochondrial protein caseinolytic protease CLPX-CLPP complex (CLPXP). A mutation in CLPX has been linked to the disorder erythropoietic protoporphyria, but the mechanism by which heme induces ALAS degradation had not been described. RATIONALE: We hypothesized that an adaptor protein targets heme-bound ALAS for degradation by CLPXP. We therefore investigated polymerase delta-interacting protein 2 (POLDIP2), a candidate adaptor protein, in a reconstituted biochemical system and validated the relevance of our findings to the control of heme biosynthesis in erythroid cells, in which hemoglobin synthesis creates a large demand for heme. RESULTS: On the basis of existing proteomic data, we hypothesized that POLDIP2, a direct interactor of CLPX, provides the missing link between heme-bound ALAS and its degradation by CLPXP. We observed degradation of ALAS in a reconstituted system with CLPXP, POLDIP2, and heme. Both heme and POLDIP2 were required for degradation, confirming that POLDIP2 specifically induces heme-dependent degradation of ALAS. Using mutagenesis guided by the predicted structure of a complex between ALAS, POLDIP2, and heme, we determined that POLDIP2 directly binds a heme-occupied motif in the unstructured N terminus of ALAS, recruiting heme-bound ALAS to CLPXP. Heme absorbance spectra showed that residues from both ALAS and POLDIP2 at their interface are required to coordinate the heme iron, suggesting that heme acts as a molecular glue for this complex. After heme-dependent complex formation, CLPXP required access to the C terminus of ALAS to degrade ALAS, indicating this site as the initiation point for unfolding. In human embryonic kidney (HEK293) cells and in the mouse erythroleukemia (MEL) cell line, we observed that deletion of POLDIP2 blocked degradation of both the housekeeping and erythroid isozymes of ALAS, establishing the physiological importance of this regulatory mechanism. In MEL cells, POLDIP2 loss led to accumulation of the final heme precursor, protoporphyrin IX (PPIX), recapitulating the phenotype of XLPP, an erythropoietic protoporphyria caused by truncation of the ALAS2 C-terminal element. In MEL cells, both POLDIP2 loss and a previously identified XLPP allele also led to accumulation of ALAS2. CONCLUSION: This study identifies POLDIP2 as a critical link in the feedback regulation of heme biosynthesis. POLDIP2 presents heme-bound ALAS to CLPX, after which CLPX engages the C-terminal extension of ALAS for unfolding and degradation by CLPP. These data establish a mechanism that underlies erythropoietic protoporphyrias linked to CLPX and ALAS2 and indicate that variation in POLDIP2 may cause or modify erythropoietic or hepatic porphyrias. The constitutive association of POLDIP2 with CLPX and its conservation in heme auxotrophs suggest that variations of the substrate-delivery mechanism we describe may be used for a broader set of substrates to tune mitochondrial functions through proteolysis. An adaptor protein orchestrates heme-induced protein degradation to limit heme biosynthesis through a negative feedback loop.: POLDIP2 senses heme occupancy of a motif in ALAS, the first enzyme in heme biosynthesis, to commit ALAS for degradation by the mitochondrial protease CLPXP. This mechanism couples heme biosynthesis directly to cellular need and prevents accumulation of toxic PPIX. [ABSTRACT FROM AUTHOR] |
| Copyright of Science is the property of American Association for the Advancement of Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.) | |
| Database: | Psychology and Behavioral Sciences Collection |
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| Items | – Name: Title Label: Title Group: Ti Data: An adaptor for feedback regulation of heme biosynthesis by a mitochondrial protease. – Name: Author Label: Authors Group: Au Data: <searchLink fieldCode="AR" term="%22Cottle%2C+Thomas%22">Cottle, Thomas</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Joh%2C+Lydia%22">Joh, Lydia</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Posner%2C+Cori%22">Posner, Cori</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22DeCosta%2C+Adam%22">DeCosta, Adam</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Campagna%2C+Dean+R%2E%22">Campagna, Dean R.</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Fleming%2C+Mark+D%2E%22">Fleming, Mark D.</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Ducamp%2C+Sarah%22">Ducamp, Sarah</searchLink> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Kardon%2C+Julia+R%2E%22">Kardon, Julia R.</searchLink> (AUTHOR) – Name: TitleSource Label: Source Group: Src Data: <searchLink fieldCode="JN" term="%22Science%22">Science</searchLink>. 7/30/2026, Vol. 393 Issue 6810, p1-9. 9p. – Name: Abstract Label: Abstract Group: Ab Data: Heme biosynthesis is tightly coordinated to support essential functions without accumulating toxic porphyrins and depleting cellular iron. Heme induces degradation of the heme biosynthetic enzyme, 5-aminolevulinate synthase (ALAS), by the mitochondrial caseinolytic protease complex CLPX-CLPP (CLPXP), but the mechanism for heme-triggered degradation had not been elucidated. We found that polymerase delta-interacting protein 2 (POLDIP2) is a heme-sensing adaptor protein sufficient to reconstitute negative feedback degradation of ALAS by CLPXP. POLDIP2 was necessary to support ALAS turnover in cells and regulate heme production during erythropoiesis. POLDIP2 directly recognized and recruited heme-bound ALAS to CLPXP. Degradation initiation required a carboxyl-terminal element of ALAS, truncations of which cause an erythropoietic protoporphyria. Our findings establish a mechanism for conditional degradation by CLPXP that underlies erythropoietic protoporphyrias linked to CLPX and ALAS. Editor's summary: The molecule heme has important functions in oxygen transport, as a cofactor for enzymes, and as a sensor. It must be available as needed, but excessive synthesis can cause the accumulation of toxic precursors or depletion of iron, so its production is tightly controlled. The unfoldase protein CLPX activates 5-aminolevulinate synthase (ALAS), the rate-limiting enzyme for heme synthesis, but balances this activity as part of the proteolytic complex CLPXP, which degrades ALAS isozymes. Cottle et al. report that this switch requires polymerase delta-interacting protein 2 (POLDIP2). This protein binds heme-bound ALAS and acts as an adaptor that promotes CLPXP-dependent degradation of ALAS and may thus help to modulate heme production during erythropoiesis. —L. Bryan Ray INTRODUCTION: Biosynthesis of heme is tightly coordinated with cellular demand, such that vital functions of this essential cofactor, including oxygen transport, respiration, and catalysis, are fully supplied without accumulating toxic heme precursors or depleting cellular iron. Heme biosynthesis begins in the mitochondrial matrix with the rate-limiting enzyme 5-aminolevulinate synthase (ALAS). One mechanism through which ALAS activity is controlled is by negative feedback degradation triggered by a heme-binding site in ALAS. This degradation requires the mitochondrial protein caseinolytic protease CLPX-CLPP complex (CLPXP). A mutation in CLPX has been linked to the disorder erythropoietic protoporphyria, but the mechanism by which heme induces ALAS degradation had not been described. RATIONALE: We hypothesized that an adaptor protein targets heme-bound ALAS for degradation by CLPXP. We therefore investigated polymerase delta-interacting protein 2 (POLDIP2), a candidate adaptor protein, in a reconstituted biochemical system and validated the relevance of our findings to the control of heme biosynthesis in erythroid cells, in which hemoglobin synthesis creates a large demand for heme. RESULTS: On the basis of existing proteomic data, we hypothesized that POLDIP2, a direct interactor of CLPX, provides the missing link between heme-bound ALAS and its degradation by CLPXP. We observed degradation of ALAS in a reconstituted system with CLPXP, POLDIP2, and heme. Both heme and POLDIP2 were required for degradation, confirming that POLDIP2 specifically induces heme-dependent degradation of ALAS. Using mutagenesis guided by the predicted structure of a complex between ALAS, POLDIP2, and heme, we determined that POLDIP2 directly binds a heme-occupied motif in the unstructured N terminus of ALAS, recruiting heme-bound ALAS to CLPXP. Heme absorbance spectra showed that residues from both ALAS and POLDIP2 at their interface are required to coordinate the heme iron, suggesting that heme acts as a molecular glue for this complex. After heme-dependent complex formation, CLPXP required access to the C terminus of ALAS to degrade ALAS, indicating this site as the initiation point for unfolding. In human embryonic kidney (HEK293) cells and in the mouse erythroleukemia (MEL) cell line, we observed that deletion of POLDIP2 blocked degradation of both the housekeeping and erythroid isozymes of ALAS, establishing the physiological importance of this regulatory mechanism. In MEL cells, POLDIP2 loss led to accumulation of the final heme precursor, protoporphyrin IX (PPIX), recapitulating the phenotype of XLPP, an erythropoietic protoporphyria caused by truncation of the ALAS2 C-terminal element. In MEL cells, both POLDIP2 loss and a previously identified XLPP allele also led to accumulation of ALAS2. CONCLUSION: This study identifies POLDIP2 as a critical link in the feedback regulation of heme biosynthesis. POLDIP2 presents heme-bound ALAS to CLPX, after which CLPX engages the C-terminal extension of ALAS for unfolding and degradation by CLPP. These data establish a mechanism that underlies erythropoietic protoporphyrias linked to CLPX and ALAS2 and indicate that variation in POLDIP2 may cause or modify erythropoietic or hepatic porphyrias. The constitutive association of POLDIP2 with CLPX and its conservation in heme auxotrophs suggest that variations of the substrate-delivery mechanism we describe may be used for a broader set of substrates to tune mitochondrial functions through proteolysis. An adaptor protein orchestrates heme-induced protein degradation to limit heme biosynthesis through a negative feedback loop.: POLDIP2 senses heme occupancy of a motif in ALAS, the first enzyme in heme biosynthesis, to commit ALAS for degradation by the mitochondrial protease CLPXP. This mechanism couples heme biosynthesis directly to cellular need and prevents accumulation of toxic PPIX. [ABSTRACT FROM AUTHOR] – Name: AbstractSuppliedCopyright Label: Group: Ab Data: <i>Copyright of Science is the property of American Association for the Advancement of Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.) |
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| RecordInfo | BibRecord: BibEntity: Identifiers: – Type: doi Value: 10.1126/science.ads5397 Languages: – Code: eng Text: English PhysicalDescription: Pagination: PageCount: 9 StartPage: 1 Titles: – TitleFull: An adaptor for feedback regulation of heme biosynthesis by a mitochondrial protease. Type: main BibRelationships: HasContributorRelationships: – PersonEntity: Name: NameFull: Cottle, Thomas – PersonEntity: Name: NameFull: Joh, Lydia – PersonEntity: Name: NameFull: Posner, Cori – PersonEntity: Name: NameFull: DeCosta, Adam – PersonEntity: Name: NameFull: Campagna, Dean R. – PersonEntity: Name: NameFull: Fleming, Mark D. – PersonEntity: Name: NameFull: Ducamp, Sarah – PersonEntity: Name: NameFull: Kardon, Julia R. IsPartOfRelationships: – BibEntity: Dates: – D: 30 M: 07 Text: 7/30/2026 Type: published Y: 2026 Identifiers: – Type: issn-print Value: 00368075 Numbering: – Type: volume Value: 393 – Type: issue Value: 6810 Titles: – TitleFull: Science Type: main |
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