Calcineurin B-like (CBL) proteins represent a unique family of calcium sensors in plant cells. Sensing the calcium signals elicited by a variety of abiotic stresses, CBLs transmit the information to a group of serine/threonine protein kinases (CBLinteracting protein kinases [CIPKs]), which are currently known as the sole targets of the CBL family. Here, we report that the CBL3 member of this family has a novel interaction partner in addition to the CIPK proteins. Extensive yeast two-hybrid screenings with CBL3 as bait identified an interesting Arabidopsis (Arabidopsis thaliana) cDNA clone (named AtMTAN, for 5#-methylthioadenosine nucleosidase), which encodes a polypeptide similar to EcMTAN from Escherichia coli. Deletion analyses showed that CBL3 utilizes the different structural modules to interact with its distinct target proteins, CIPKs and AtMTAN. In vitro and in vivo analyses verified that CBL3 and AtMTAN physically associate only in the presence of Ca 2+ . In addition, we empirically demonstrated that the AtMTAN protein indeed possesses the MTAN activity, which can be inhibited specifically by Ca 2+ -bound CBL3. Overall, these findings suggest that the CBL family members can relay the calcium signals in more diverse ways than previously thought. We also discuss a possible mechanism by which the CBL3-mediated calcium signaling regulates the biosynthesis of ethylene and polyamines, which are involved in plant growth and development as well as various stress responses. Plant cells use calcium ion (Ca

Poster Sessions 4 4 binds. Neither the extended conformation of the donor analogue nor the induced fit it causes in the protein have been observed before in an rGT. Comparison with the previously determined binary and ternary complexes of Kre2p/Mnt1p with its GDP donor product reveals that the GDP moiety of the GDP-2F-Man is bound in a similar manner. However, its β-phosphate group is in a different position, consistent with the attached 2-fluoro-mannose moiety being buried in the pocket. A triad of charged residues, E247, R245 and D361, is involved in the formation of the pocket due to conformational changes of the R245 and D361 side chains. The carboxylate group of E247 is 3.3 Å from the reactive centre, the β-face of the C1 group of mannose, suggesting its involvement in the reaction mechanism. When the donor analogue is bound, R245 no longer interacts with E247, making E247 even more catalytically relevant. Two triad residues have been shown to be essential for catalysis by sitedirected mutagenesis, and all are structurally conserved in most rGT structures, which suggests that the proposed catalytic mechanism relying on the concerted action of charged triad residues could potentially be a common mechanism for most rGTs.