PT  - JOURNAL ARTICLE
AU  - Aarthi Putarjunan
AU  - Jim Ruble
AU  - Ashutosh Srivastava
AU  - Chunzhao Zhao
AU  - Amanda L. Rychel
AU  - Alex K. Hofstetter
AU  - Xiaobo Tang
AU  - Jian-Kang Zhu
AU  - Florence Tama
AU  - Ning Zheng
AU  - Keiko U. Torii
TI  - Bipartite anchoring of SCREAM enforces stomatal initiation by coupling MAP Kinases to SPEECHLESS
AID  - 10.1101/587154
DP  - 2019 Jan 01
TA  - bioRxiv
PG  - 587154
4099  - http://biorxiv.org/content/early/2019/03/24/587154.short
4100  - http://biorxiv.org/content/early/2019/03/24/587154.full
AB  - Cell-fate in eukaryotes is regulated by MAP Kinases (MAPKs) that translate external cues to cellular responses. In plants, two MAPKs, MPK3/6, regulate diverse processes of development, environmental response, and immunity. Yet, the mechanism bridging these shared signaling components with a specific target remains unresolved. Focusing on the development of stomata, epidermal valves for gas exchange and transpiration, we report here that the bHLH protein SCREAM functions as a scaffold by recruiting MPK3/6 to downregulate SPEECHLESS, a transcription factor initiating stomatal cell lineages. SCREAM directly binds with MPK3/6 through an evolutionarily-conserved yet unconventional bipartite motif. Mutations in this motif abrogate association, phosphorylation and degradation of SCREAM, unmask hidden non-redundancies between MPK3 and MPK6, and result in uncontrolled stomatal differentiation. Structural analyses of MPK6 at the 2.75Å resolution unraveled bipartite binding of SCREAM with MPK6, that is distinct from an upstream MAPKK. Our findings elucidate, at the atomic resolution, the mechanism directly linking extrinsic signals to transcriptional reprogramming during the establishment of stomatal cell-fate, and highlight a unique substrate-binding mode adopted by plant MAPKs.