Proteomic analysis of the endoplasmic reticulum from developing and germinating seed of castor (Ricinus communis)

Endoplasmic reticulum (ER) has been prepared and analysed from germinating and developing castor bean endosperm. A combination of one- and two-dimensional (1-D and 2-D) gel electrophoresis was used to study the complexity of sample and protein differences between the two stages. The ER of the developing oilseed is central to the synthesis, sorting and storage of protein and lipid reserves while the germinating seed is concerned with their degradation. Sample complexity has been reduced by separation of ER proteins into lumenal, peripheral membrane and integral membrane subfractions. Membrane proteins pose specific problems in aggregation and binding to passive surfaces. We have overcome this by collection of membranes at density gradient interfaces and by silanization of plastic ware. Several major components have been identified from 1-D gels by N-terminal sequencing and matrix-assisted laser desorption/ionization (MALDI) peptide mass fingerprints. These include protein disulphide isomerase (PDI), calreticulin and developing-ER-specific oleate-12-hydroxylase involved in the biosynthesis of ricinoleic acid. In excess of 300 spots are detectable in each developmental fraction by high sensitivity 2-D gels. This is the first 2-D electrophoretic analysis of plant ER. These gels reveal significant differences between germinating and developing ER. Preparative loading 2-D gels of germinating ER have been run and 14 selected spots characterized by quadrupole time of flight tandem mass spectrometry (Q-TOF MS/MS). Ten of these proteins were assigned function on the basis of identity with existing castor database entries, or by homology with other species. Two proteins, aspartate proteinase precursor and N-carbamyl-L-aminohydrolase-like protein, appear to be absent from developing profiles. Most of the proteins identified are concerned with roles in protein processing and storage, and lipid metabolism which occur in the ER. Data from three of the assigned spots included unidentified peptides indicating the presence of more than one protein in these spots following 2-D electrophoresis. More extensive analysis will have to await developments in genomics but the basic separation technologies to simplify sample identity for a plant ER preparation have been established.