Respiratory and C4-photosynthetic NAD-malic enzyme coexist in bundle sheath cells mitochondria and evolved via association of differentially adapted subunits

In Cleomaceae species, NAD-malic enzyme (NAD-ME) was independently co-opted to participate in C4 photosynthesis. In C4 Cleome species, all NAD-ME genes (NAD-MEα, -ß1 and -ß2) were affected by C4 evolution and are expressed at higher levels than their C3 orthologs in Tarenaya hassleriana. In C3 Cleome, the NAD-ME housekeeping function is performed by two heteromers, NAD-MEα/ß1 and NAD-MEα/ß2, with similar biochemical properties and tissue occurrence. In the C4 species, Gynandropsis gynandra and Cleome angustifolia, this role is performed only by the NAD-MEα/ß2 heteromer. In these C4 species NAD-MEα/ß1 is preferentially present in leaves of the C4 species where is the predominant isoform. GgNAD-MEα/ß1 exhibits high catalytic efficiency and is differentially activated by the C4 intermediate aspartate. In C4 Cleome NAD-MEα/ß1 represents thus the C4-decarboxylase. GgNAD-MEß1and CaNAD-MEß1 are non-catalytic subunits but impart a stabilizing effect on the associated α-subunit. We conclude that in C4 Cleome the functions of NAD-ME as a TCA cycle associated enzyme and as a C4 photosynthetic decarboxylase coexist in BSC mitochondria and are performed by isoforms originated through associations of differentially adapted subunits.