NITRATE-INDUCED POLYPEPTIDES IN MEMBRANES FROM CORN SEEDLING ROOTS

The polypeptide composition of the membranes from corn (Zea mays L.) s eedling roots upon nitrate induction was determined by two-dimensional gel electrophoresis and silver-staining. the synthesis of five polype ptides (49, 48, 35, 33, and 32 kDa) in the tono-plast fraction and fou r polypeptides (50, 49, 33, and 33 kDa) in the plasma membrane fractio n was induced by both 2.5 mM Ca(NO3)(2) and 5 mM KNO3. Extensive washi ng of the membranes with salt and NaOH demonstrated that three induced polypeptides (49, 48, and 35 kDa) in the tonoplast fraction and two i nduced polypeptides (49 and 33 kDa) in the plasma membrane fraction we re integral proteins. After incubation of seedlings in N-free medium f or 4 d, the 49 and 32 kDa polypeptides in the tonoplast fraction had d isappeared. By the sixth day in N-free medium, the 35 kDa polypeptide had disappeared from the tonoplast fraction. the 50 kDa polypeptide of the plasma membrane fraction was no longer detectable in seedlings in cubated for 6 d in N-free medium. The size of the spots corresponding to the 33 kDa polypeptides of both membrane fractions and to the 49 kD a polypeptide of the plasma membrane fraction was reduced following in cubation of seedlings in N-free medium. The changes in nitrate-induced polypeptides in both membrane fractions following transfer to N-free medium correlated with a reduced capacity to take up nitrate in the tr eated seedlings. The results support the conclusion that the nitrate-i nduced polypeptides may be involved in nitrate transport across the to noplast and plasma membrane.