PROTEIN-PROTEIN INTERACTIONS IN NON-PHOTOSYNTHETIC PLASTIDS OF ROOTS AND FLOWER, AND CHLOROPLASTS IN C3 AND C4 PLANTS

¹ Department of Plant Biology, Cornell University, Ithaca, NY 14853, USA

² Computational Biology Service Unit, Cornell University, Ithaca, NY 14853, USA

Plastids are essential organelles of prokaryotic origin that are present in every plant cell and differentiate from proplastids into non-photosynthetic plastids in roots and flowers and chloroplasts in leafs and stems. In C4 plants such as maize, plastids also differentiate within the leaf, with specialized bundle sheath (BS) chloroplasts in the BS cells surrounding the vasculature and the mesophyll (M) chloroplasts in M cells lying adjacent to vasculature. Ten to twelve percent of the ~29,500 Arabidopsis thaliana genes are predicted to encode for plastid proteins, underlining the importance of this organelle for the plant cell. We report on the investigation of the proteomes of non-photosynthetic plastids in roots and flowers and leaf chloroplasts in C3 and C4 plants. This will allow to better define the different plastid types and to determine their specific roles. In addition, we focus on identification of protein-protein interactions, since this is an efficient way to determine their biochemical function, as will be demonstrated by several examples. For instance, we identified a ~350 kDa Clp protease complex, with 11 different gene products in non-photosynthetic plastids of roots, flower petals and in chloroplasts of leaves of Arabidopsis thaliana. These complexes were purified based on native mass and isoelectric point and identified by mass spectrometry. The stoichiometry between the subunits was also determined and 3-D homology modeling showed that the identified proteins fit well together in a tetradecameric complex and also indicated unique contributions for each protein.