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.
Böhm et al. (Thu,) studied this question.