A milestone in C4 carbon concentration mechanism evolution: structural remodeling of NADP-malic enzyme in Poaceae

Le résumé fourni par la source

The evolution of C4 photosynthesis required extensive modification of ancestral enzymes enabling the development of anefficient carbon concentrating mechanism. A key example is NADP-malic enzyme (NADP-ME), which, in maize and sorghum—members of the same C4 lineage—underwent gene duplication and neofunctionalization, resulting in 2 plastidic isoformswith distinct oligomeric states: a tetrameric C4-specific isoform and a dimeric housekeeping (nonC4) isoform. In thisstudy, we resolve the structural basis of this oligomeric divergence using X-ray crystallography, cryo-electronmicroscopy, and molecular modeling combined with targeted biochemical analysis. Our findings demonstrate that theN-terminal region of nonC4-NADP-ME is involved in its oligomeric organization, whereas a suite of adaptive substitutionsat the dimer interface drives the transition to the stable tetramer characteristic of the C4 isoform. Moreover, theC-terminal region stabilizes the oligomeric states of C4- and nonC4-NADP-ME through specific interactions with adaptiveresidues. We propose that tetramerization mitigates aggregation at the high expression levels demanded by the C4 cycleand likely creates a scaffold for the emergence of regulatory properties. Collectively, the data show that remodeling ofterminal domains and inter-subunit interfaces rewires the quaternary architecture of the enzymes, illustrating howsubtle structural changes can drive the evolution of complex innovations such as C4 photosynthesis.

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