Differential Protein Composition and Gene Expression in Leaf Mesophyll Cells and Bundle Sheath Cells of the C4 Plant Digitaria sanguinalis (L.) Scop.

Abstract

The distribution and molecular weights of cellular proteins in soluble and membrane-associated locations were analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Coomassie blue staining of leaf (D. sanguinalis L. Scop.) extracts and isolated cell extracts. Leaf polypeptides also were pulse-labeled, followed by isolation of the labeled leaf cell types and analysis of the newly synthesized polypeptides in each cell type by electrophoresis and fluorography. Comparison of the electrophoretic patterns of crabgrass whole leaf polypeptides with isolated cell-type polypeptides indicated a difference in protein distribution patterns for the 2 cell types. The mesophyll cells exhibited a greater allocation of total cellular protein into membrane-associated proteins relative to soluble proteins. In contrast, the bundle sheath cells exhibited a higher percentage of total cellular protein in soluble proteins. Phosphoenolpyruvate carboxylase was the major soluble protein in the mesophyll cell and ribulose bisphosphate carboxylase was the major soluble protein in the bundle sheath cell. The majority of in vivo 35S-pulse-labeled proteins synthesized by the 2 crabgrass cell types corresponded in molecular weight to the proteins present in the cell types which were detected by conventional staining techniques. The bundle sheath cell and mesophyll cell fluorograph profiles each had 15 major 35S-labeled proteins. The major incorporation of 35S by bundle sheath cells was into products which co-electrophoresed with the large and small subunits of ribulose bisphosphate carboxylase. In contrast, a major 35S-labeled product in mesophyll cell extracts co-electrophoresed with the subunit of phosphoenolpyruvate carboxylase. Both cell types exhibited equivalent in vivo labeling of a polypeptide with one- and two-dimensional electrophoretic behavior similar to the major apoprotein of the light-harvesting chlorophyll a/b protein. Results from the use of protein synthesis inhibitors during pulse-labeling experiments indicated intercellular differences in both organelle and cytoplasmic protein synthesis. A majority of the 35S incorporation by crabgrass mesophyll cell 70S ribosomes was associated with a pair of membrane-associated polypeptides of MW 32,000 and 34,500; a comparison of fluorograph and stained gel profiles suggest these products resemble the precursor and mature forms of the maize chloroplast 32,000 dalton protein reported by Grebanier et al (1978). In contrast, crabgrass bundle sheath cell organelle translocation was directed predominantly into a product which coelectrophoresed with the large subunit of ribulose bisphosphate carboxylase.