The research history of the auxin-binding protein ABP1 is convoluted and serves as an excellent example of nonlinear scientific progress. It was a promising extracellular auxin receptor in the long ‘Antique period’, roughly between the 1970s and 1990s. The ‘Dark Ages’ started in 2005 after interest shifted owing to the discovery of TIR1 as a nuclear auxin receptor linked to the major auxin-induced transcriptional responses. An absolute all-time low came around 2015, when it was shown that important phenotypes attributed to ABP1 were due to off-target and artefactual effects. But that was not the end: the ‘Renaissance’ suddenly happened in 2022 as some robust but subtle phenotypes were assigned to ABP1 and its docking partners, the transmembrane kinases (TMKs). Now, in a breakthrough report in Cell, Tongda Xu, Zhenbiao Yang and colleagues demonstrate that the lack of a strong visible phenotype in the abp1 mutant was due to genetic redundancy. Two functional homologues of ABP1, named ABP1-like (ABL) proteins, participate in the same pathways with overlapping functions.
After showing that a dominant-negative version of ABP1 can suppress known TMK-linked auxin responses (which suggested the presence of yet unknown redundant partners), the authors characterized proteins that directly bind to the extracellular domain of TMK1. They found two secreted germin-like proteins localized in the apoplast that had low sequence but high structural homology with ABP1: ABL1 and ABL2. Double and triple mutants showed increasingly obvious auxin-related growth phenotypes, and complementation experiments indicated overlapping and distinct functions for each of the proteins. Comprehensive genetic and biochemical assays proved that TMKs and the three auxin-binding proteins all interact with auxin. The intracellular kinase domain of the TMKs then transmits the signal across the membrane to various downstream pathways by phosphorylation, which explains most of the fast non-transcriptional responses to auxin.
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