TTL proteins scaffold brassinosteroid signaling components at the plasma membrane to optimize signal transduction in Arabidopsis

Brassinosteroids (BRs) form a group of steroidal hormones essential for plant growth, development and stress responses. BRs are perceived extracellularly by plasma membrane receptor-like kinases, which activates an interconnected signal transduction cascade, leading to the transcriptional regulation of BR-responsive genes. TETRATRICOPEPTIDE THIOREDOXIN-LIKE (TTL) genes are specific for land plants and their encoded proteins are defined by the presence of protein-protein interaction motives, i.e. an intrinsic disordered region at the N-terminus, six tetraticopeptide repeat domains and a C-terminus with homology to thioredoxins, thus likely mediating the assembly of multiprotein complexes. Phenotypic, molecular and genetic analyses show that TTL proteins are positive regulators of BR signaling in Arabidopsis. TTL3 directly interacts with a constitutively active BRI1 receptor kinase, BSU1 phosphatase and the BZR1 transcription factor and associates with BSK1, BIN2 kinases but not with BAK1. A functional TTL3-GFP shows dual cytoplasmic-plasma membrane localization. Depleting the endogenous BR content reduces plasma membrane localization of TTL3-GFP, while increasing BR content causes its plasma membrane relocalization, where it strengthens the association of BR signaling components. Our results reveal that TTL proteins promote BR responses and suggest that may function as scaffold proteins by bringing together cytoplasmic and plasma membrane BR signaling components.