Calcineurin B-like Protein CBL10 Directly Interacts with TOC34 (Translocon of the Outer Membrane of the Chloroplasts) and Decreases Its GTPase Activity in Arabidopsis

As calcium sensor relays in plants, calcineurin B-like (CBL) proteins provide an important contribution to decoding Ca(2+) signatures elicited by a variety of abiotic stresses. Currently, it is well known that CBLs perceive and transmit the Ca(2+) signals mainly to a group of serine/threonine protein kinases called CBL-interacting protein kinases (CIPKs). In this study, we report that the CBL10 member of this family has a novel interaction partner besides the CIPK proteins. Yeast two-hybrid screening with CBL10 as bait identified an Arabidopsis cDNA clone encoding a TOC34 protein, which is a member of the TOC (Translocon of the Outer membrane of the Chloroplasts) complex and possesses the GTPase activity. Further analyses showed that in addition to CBL10, CBL7 also interacts with TOC34 at much lower strength in the yeast two-hybrid system. However, the rest of the CBL family members failed to interact with TOC34. Bimolecular fluorescence complementation (BiFC) analysis verified that the CBL10-TOC34 interaction occurs at the outer membrane of chloroplasts in vivo. In addition, we also demonstrated that CBL10 physically associates with TOC34 in vitro, resulting in a significant decrease in the GTPase activity of the TOC34 protein. Taken together, our findings clearly indicate that a member of the CBL family, CBL10, can modulate not only the CIPK members but also TOC34, allowing the CBL family to relay the Ca(2+) signals in more diverse ways than currently known.