TTL Proteins Scaffold Brassinosteroid Signaling Components at the Plasma Membrane to Optimize Signal Transduction in Plant Cells

Brassinosteroids (BRs) form a group of steroidal hormones essential for plant growth, development and stress responses. BR perception at the plasma membrane initiates a series of phosphorylation events enabling the nuclear accumulation and activity of the key transcription factors BZR1 and BES1. Previous studies support that all BR signaling components form an interconnected pathway, although it is not known how these proteins are brought together for the prompt signal transduction from the plasma membrane to the nucleus. Here we report that plant-specific Tetratricopeptide Thioredoxin- Like (TTL) proteins are positive regulators of BR signaling that function as scaffold for the BR signaling components in Arabidopsis. TTL3 directly interacts with BZR1 and the phosphatase BSU1, and associates in vivo with most core components involved in BR signaling, with the exception of the BAK1 coreceptor. TTL3 is mainly localized in the cytoplasm, and BR treatment increases its localization at the plasma membrane. In addition, the expression of TTL3 strengthens the association of BR-signaling components BSK1 and BZR1 at the plasma membrane. Consistent with a role in BR signaling, mutations in TTL3, and related TTL1 and TTL4 genes cause reduced BR responses, and these defects that highly enhanced in a triple ttl1 ttl3 ttl4 mutant. We propose a novel mechanistic model for BR signaling, in which cytoplasmic/nuclear BR components bound to TTL proteins are recruited to the plasma membrane upon BR perception, which in turn allows the assembly of a BR signaling complex with the goal of ensuring dephosphorylation and nuclear accumulation of the transcription factors BZR1 and BES1. Interestingly, this novel TTL scaffold model for BR signaling resembles that of Wnt signaling in metazoans, in which TTL proteins would act similar to Axin1, optimizing signaling efficiency of the cascade by promoting the assembly of the signaling complex at the plasma membrane.