S-35 METHIONINE INCORPORATES DIFFERENTIALLY INTO POLYPEPTIDES ACROSS LEAVES OF SPINACH (SPINACIA-OLERACEA)

The distribution of proteins across leaves may have significant impact on optimal photosynthetic performance of leaves, however little is kn own about the distribution of proteins and protein synthesis across C- 3 leaves. We report here a detailed investigation of S-35-methionine i ncorporation into polypeptides and the steady-state polypeptide profil es at different leaf depths across spinach leaves. About 10 highly inc orporating polypeptides (three with apparent molecular masses of 23 kD a, 21 kDa and 17 kDa were especially dominant) were detected in a few medial leaf sections. These highly incorporating polypeptides were sol uble proteins, except for the 17 kDa polypeptide, which was associated with thylakoid membranes. All of the highly incorporating polypeptide s were nuclearly encoded. Light significantly enhanced S-35-methionine incorporation into the highly incorporating polypeptides in ''sun'' g rown leaves, but not in ''shade'' grown leaves. Microautoradiography s howed that the highly incorporating polypeptides were associated mainl y with the phloem tissue. A specific identity or function for the poly peptides is not known. The concentration of most polypeptides on an ar eal basis appeared to increase with leaf depth from the adaxial leaf s urface, reaching a maximum around 25% of the leaf depth, and then decl ined gradually towards the abaxial surface. The periphery of cells exh ibited high levels of S-35-methionine incorporation, and microautoradi ography showed that the label was mainly located in the symplast. In g eneral, polypeptides exhibited higher rates of S-35-methionine incorpo ration in the palisade mesophyll than in the spongy mesophy, probably due to cytoplasmic density and light. The data show that it may be pos sible to study vascular bundle proteins using paradermal leaf sections . In addition, we now can investigate how factors such as light or CO2 might control protein distribution across leaves, and further explore the complex interactions among photosynthesis, leaf anatomy, and ligh t.