Nitrate-induced polypeptides in membranes from corn seedling roots

The polypeptide composition of the membranes from corn (Zea mays L.) seedling roots upon nitrate induction was determined by two-dimensional gel electrophoresis and silver-staining. The synthesis of five polypeptides (49, 48, 35, 33, and 32 kDa) in the tono-plast fraction and four polypeptides (50, 49, 38, and 33 kDa) in the plasma membrane fraction was induced by both 2.5 mM Ca(NO₃)₂ and 5 mM KNO₃. Extensive washing of the membranes with salt and NaOH demonstrated that three induced polypeptides (49, 48, and 35 kDa) in the tonoplast fraction and two induced polypeptides (49 and 33 kDa) in the plasma membrane fraction were integral proteins. After incubation of seedlings in N-free medium for 4 d, the 49 and 32 kDa polypeptides in the tonoplast fraction had disappeared. 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 incubated 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 kDa polypeptide of the plasma membrane fraction was reduced following incubation 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 treated seedlings. The results support the conclusion that the nitrate-induced polypeptides may be involved in nitrate transport across the tonoplast and plasma membrane.