Crystal structure of potato 14-3-3 protein St14f revealed the importance of helix I in StFDL1 recognition.

In potato (Solanum tuberosum L.), 14-3-3 protein forms a protein complex with the FLOWERING LOCUS T (FT)-like protein StSP6A and the FD-like protein StFDL1 to activate potato tuber formation. Eleven 14-3-3 isoforms were reported in potato, designated as St14a-k. In this study, the crystal structure of the free form of St14f was determined at 2.5 Å resolution. Three chains were included in the asymmetric unit of the St14f free form crystal, and the structural deviation among the three chain structures was found on the C-terminal helix H and I. The St14f free form structure in solution was also investigated by nuclear magnetic resonance (NMR) residual dipolar coupling analysis, and the chain B in the crystal structure was consistent with NMR data. Compared to other crystal structures, St14f helix I exhibited a different conformation with larger B-factor values. Larger B-factor values on helix I were also found in the 14-3-3 free form structure with higher solvent contents. The mutation in St14f Helix I stabilized the complex with StFDL1. These data clearly showed that the flexibility of helix I of 14-3-3 protein plays an important role in the recognition of target protein.

Potato Tuber Induction is Regulated by Interactions Between Components of a Tuberigen Complex.

Photoperiod-regulated flowering and potato tuber formation involve leaf-produced mobile signals, florigen and tuberigen, respectively. The major protein component of florigen has been identified as the FLOWERING LOCUS T (FT) protein. In rice, an FT-like protein, Heading date 3a (Hd3a), induces flowering by making the florigen activation complex (FAC) through interactions with 14-3-3 and OsFD1, a rice FD-like protein. In potato, StSP6A, an FT-like protein, was identified as a major component of tuberigen. However, the molecular mechanism of how StSP6A triggers tuber formation remains elusive. Here we analyzed the significance of the formation of a complex including StSP6A, 14-3-3 and FD-like proteins in tuberization. Yeast two-hybrid, bimolecular fluorescence complementation and in vitro pull-down assays showed that StSP6A and StFDL1, a potato FD-like protein, interact with St14-3-3s. StSP6A overexpression induced early tuberization in a 14-3-3-dependent manner, and suppression of StFDL1 delayed tuberization. These results strongly suggest that an FAC-like complex, the tuberigen activation complex (TAC), comprised of StSP6A, St14-3-3s and StFDL1, regulates potato tuber formation.