Parallels Between Animal Raf1 and Plant SPAK

Abstract

Shoot architecture and flowering are regulated processes and vary among plant species. Pnueli et al. identified a new group of interacting partners, SP-interacting proteins (SIPs), for the CETS family of proteins (so named for the founding members CEN, TFL1, and FT, which are all regulators of flowering and shoot architecture). CETS proteins are homologous to the mammalian phosphatidylethanolamine-binding proteins; however, analysis of the crystal structure of CEN suggests that these proteins do not bind phospholipid, but may bind phosphorylated amino acids instead. In a screen for partners for the CETS member SP, Pnueli et al. identified several 14-3-3 proteins, a kinase SPAK (SP-associated kinase), a G-box transcription factor of the bZIP family, and a novel small protein SIP4. Their analysis of the interactions and phosphorylation dependence of the interactions revealed similarities with the kinase Raf1 signaling and regulatory processes. Raf1 and SPAK both have two 14-3-3 binding sites and both kinases are dependent on phosphorylation for their interactions with the 14-3-3 proteins. Each interacts with a downstream member of the CETS family: Raf1 interacts with RKIP and SPAK interacts with SP. The novel protein SIP4 is also a substrate for the SAPK in vitro and the interaction of SIP4 with 14-3-3 is also dependent on phosphorylation. Many questions remain about the organization of the two systems and the functional consequences of the interactions, but the similarities between the two systems allow targeted questions to be asked based on the information known in the each model. L. Pnueli, T. Gutfinger, D. Hareven, O. Ben-Naim, N. Ron, N. Adir, E. Lifschitz, Tomato SP-interacting proteins define a conserved signaling system that regulates shoot architecture and flowering. Plant Cell 13 , 2687-2702 (2001). [Abstract] [Full Text]