The Distinct Functional Roles of the Inner and Outer Chloroplast Envelope of Pea (Pisum sativum) As Revealed by Proteomic Approaches

Protein
profiles of inner (IE) and outer (OE) chloroplast envelope
membrane preparations from pea were studied using shotgun nLC–MS/MS
and two-dimensional electrophoresis, and 589 protein species (NCBI
entries) were identified. The relative enrichment of each protein
in the IE/OE pair of membranes was used to provide an integrated picture
of the chloroplast envelope. From the 546 proteins identified with
shotgun, 321 showed a significant differential distribution, with
180 being enriched in IE and 141 in OE. To avoid redundancy and facilitate in silico localization, Arabidopsis homologues were used to obtain a nonredundant list of 409 envelope
proteins, with many showing significant OE or IE enrichment. Functional
classification reveals that IE is a selective barrier for transport
of many metabolites and plays a major role in controlling protein
homeostasis, whereas proteins in OE are more heterogeneous and participate
in a wide range of processes. Data support that metabolic processes
previously described to occur in the envelope such as chlorophyll
and tocopherol biosynthesis can be ascribed to the IE, whereas others
such as carotenoid or lipid biosynthesis occur in both membranes.
Furthermore, results allow empirical assignation to the IE and/or
OE of many proteins previously assigned to the bulk chloroplast envelope
proteome.