Fusicoccin effect on the in vitro interaction between plant 14-3-3 proteins and plasma membrane H⁺-ATPase

A 17-amino acid peptide was selectively cleaved from the highly variant C terminus of the 33-kDa 14-3-3 isoform occurring in fusicoccin receptor preparations from maize and was sequenced. The determined C-terminal sequence was identical to that of the already known maize 14-3-3 homolog GF14-6, thus prompting the use of recombinant GF14-6 in an in vitro protein-protein interaction study. The cDNA of GF14-6 was expressed in Escherichia coli as a ³²P-phosphorylatable glutathione S-transferase fusion protein and was used as a probe in overlay experiments with H⁺-ATPase partially purified from maize roots. The results demonstrated that the recombinant protein specifically bound to H⁺-ATPase. The binding was dependent on Mg²⁺ and was strongly increased by fusicoccin. Controlled trypsin digestion of H⁺-ATPase abolished the association with GF14-6, a finding that was suggestive of an interaction with the C terminus of the enzyme. To confirm this result, the C- terminal domain of H⁺-ATPase was expressed as a glutathione S-transferase fusion peptide and was used in overlay experiments. GF14-6 was also able to bind to the isolated C terminus, but only in the presence of fusicoccin.