Proteomic analysis of the Arabidopsis cell wall reveals unexpected proteins with new cellular locations

We initiated a proteomic study as part of a programme aimed at discovering novel functions of the plant cell wall. Cell-wall fragments isolated from cell-suspension cultures of Arabidopsis thaliana were stripped of protein sequentially using CaCl₂ and a urea-based buffer. The protein fractions were separated by two-dimensional gel electrophoresis and individual proteins were identified by MS. We identified a number of proteins considered to be resident in other organelles but not the cell wall on the basis of their classical biological function. These included citrate synthase, which is known to be targeted to mitochondria, peroxisomes and glyoxysomes, and luminal binding protein, which is an ER (endoplasmic reticulum)-resident protein. Searches of the Arabidopsis database revealed that there are several genes encoding putative citrate synthase and luminal binding protein. We have also performed detailed analyses of the protein sequences and this paper shows how each one contains encrypted targeting information that results in the export of the protein to the extracellular matrix. We discuss the presence of alternative non-classical secretory pathways in plants.