Abstract
MATH-BTB proteins are known to act as substrate-specific adaptors of CUL3-based E3 ligases in the ubiquitin proteasome pathway. Their BTB domain binds to CUL3 scaffold proteins and the less conserved MATH domain targets a highly diverse collection of substrate proteins to promote their ubiquitination and subsequent degradation. In plants, a significant expansion of the MATH-BTB family occurred in the grasses. Here, we report analysis of TaMAB2, a MATH-BTB protein transiently expressed at the onset of embryogenesis in wheat. Due to difficulties in studying its role in zygotes and early embryos, we have overexpressed TaMAB2 in Arabidopsis to generate gain-of-function mutants and to elucidate interaction partners and substrates. Overexpression plants showed severe growth defects as well as disorganization of microtubule bundles indicating that TaMAB2 interacts with substrates in Arabidopsis. In tobacco BY-2 cells, TaMAB2 showed a microtubule and ubiquitin-associated cytoplasmic localization pattern in form of foci. Its direct interaction with CUL3 suggests functions in targeting specific substrates for ubiquitin-dependent degradation. Although direct interactions with tubulin could not be confimed, tandem affinity purification of TaMAB2 interactors point towards cytoskeletal proteins including tubulin and actin as well as the translation initiation machinery. The idenification of various subunits of eucaryotic translation initiation factors eIF3 and eIF4 as TaMAB2 interactors indicate regulation of translation initiation as a major function during onset of embryogenesis in plants.
Highlights
meprin and TRAF homology domain (MATH)-Bric à brac/tramtrack/homology domain (BTB) proteins are members of the BTB protein superfamily, encoded by the genomes of almost all eukaryotes far studied
We report about phenotypes observed in transgenic plants overexpressing Triticum aestivum MATH-BTB protein 2 (TaMAB2), the observed disorganization of microtubular structures in epidermal cells, the subcellular localization pattern of TaMAB2 during the cell cycle, its interaction with components of the cytoskeleton, with CUL3 and ubiquitin
It was previously reported that plant MATH-BTB genes cluster into a core clade and an expanded clade, which clusters into five subclades named as E1–E5 (Gingerich et al, 2007; Juranić and Dresselhaus, 2014)
Summary
MATH-BTB proteins are members of the BTB protein superfamily, encoded by the genomes of almost all eukaryotes far studied. While the Arabidopsis genome contains only six MATH-BTB genes and humans only two genes, which all encode the ancient and highly-conserved core group of MATH-BTB proteins, an expanded and highly divergent group of MATH-BTB proteins was reported for different grass species (Juranić and Dresselhaus, 2014). The core clade is comprised of MATH-BTB genes detected in all flowering plants studies including the grasses (Juranić and Dresselhaus, 2014). Due to their significant sequence conservation and constitutive expression, it was hypothesized that core-clade genes regulate ancient pathways in plant development and/or physiology (Thomas, 2006). Functional analyses of core-clade plant MATH-BTB proteins revealed their interaction with transcription factors involved in plant stress tolerance (Weber and Hellmann, 2009; Lechner et al, 2011), flowering (Chen et al, 2015), and fatty acid biosynthesis in seeds (Chen et al, 2013; Ma et al, 2013)
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