Dual Targeting to Mitochondria and Plastids of AtBT1 and ZmBT1, Two Members of the Mitochondrial Carrier Family

Zea mays and Arabidopsis thaliana Brittle 1 (ZmBT1 and AtBT1, respectively) are members of the mitochondrial carrier family. Although they are presumed to be exclusively localized in the envelope membranes of plastids, confocal fluorescence microscopy analyses of potato, Arabidopsis and maize plants stably expressing green fluorescent protein (GFP) fusions of ZmBT1 and AtBT1 revealed that the two proteins have dual localization to plastids and mitochondria. The patterns of GFP fluorescence distribution observed in plants stably expressing GFP fusions of ZmBT1 and AtBT1 N-terminal extensions were fully congruent with that of plants expressing a plastidial marker fused to GFP. Furthermore, the patterns of GFP fluorescence distribution and motility observed in plants expressing the mature proteins fused to GFP were identical to those observed in plants expressing a mitochondrial marker fused to GFP. Electron microscopic immunocytochemical analyses of maize endosperms using anti-ZmBT1 antibodies further confirmed that ZmBT1 occurs in both plastids and mitochondria. The overall data showed that (i) ZmBT1 and AtBT1 are dually targeted to mitochondria and plastids; (ii) AtBT1 and ZmBT1 N-terminal extensions comprise targeting sequences exclusively recognized by the plastidial compartment; and (iii) targeting sequences to mitochondria are localized within the mature part of the BT1 proteins.