Accumulation of 22 kDa α-zein-mediated nonzein protein in protein body of maize endosperm

Protein bodies (PBs), the major protein storage organelle in maize (Zea mays) endosperm, comprise zeins and numerous nonzein proteins (NZPs). Unlike zeins, how NZPs accumulate in PBs remains unclear. We characterized a maize miniature kernel mutant, mn*, that produces small kernels and is embryo-lethal. After cloning the Mn* locus, we determined that it encodes the mitochondrial 50S ribosomal protein L10 (mRPL10). MN* localized to mitochondria and PBs as an NZP; therefore, we renamed MN* Non-zein Protein 1 (NZP1). Like other mutations affecting mitochondrial proteins, mn* impaired mitochondrial function and morphology. To investigate its accumulation mechanism to PBs, we performed protein interaction assays between major zein proteins and NZP1, and found that NZP1 interacts with 22 kDa alpha-zein. Levels of NZP1 and 22 kDa alpha-zein in various opaque mutants were correlated. Furthermore, NZP1 accumulation in induced PBs depended on its interaction with 22 kDa alpha-zein. Comparative proteomic analysis of PBs between wild-type and opaque2 revealed additional NZPs. A new NZP with plastidial localization was also found to accumulate in induced PBs via interaction with 22 kDa alpha-zein. This study thus reveals a mechanism for accumulation of NZPs in PBs and suggests a potential application for the accumulation of foreign proteins in maize PBs.