Analysis of interactions between heterologously produced bHLH and MYB proteins that regulate anthocyanin biosynthesis: quantitative interaction kinetics by Microscale Thermophoresis.

The two Arabidopsis basic-helix-loop-helix transcription factors GLABRA3 (GL3) and ENHANCER OF GLABRA3 (EGL3) are positive regulators of anthocyanin biosynthesis, and form protein complexes (MBW complexes) with various R2R3 MYB transcription factors and a WD40 repeat protein TESTA GLABROUS1 (TTG1). In earlier studies, GL3, in contrast to EGL3, was shown to be essential for accumulation of anthocyanins in response to nitrogen depletion. This could not be fully explained by the strong induction of GL3 in response to nitrogen depletion because the EGL3 transcripts were constitutively at a relatively high level and transcripts levels of the two genes were similar under nitrogen depletion. Here the GL3 and EGL3 proteins were characterized with respect to their affinities for PRODUCTION OF ANTHOCYANIN PIGMENT2 (PAP2), a R2R3-MYB which is induced by nitrogen depletion and is part of MBW complexes promoting anthocyanin synthesis. GL3 and EGL3 were also tested for their binding to MYBL2, a negative regulator of anthocyanin synthesis and MBW complexes. Using heterologously expressed proteins and Microscale Thermophoresis, GL3 showed binding constants (Kd) of 3.5±1.7 and 22.7±3.7 μM, whereas EGL3 showed binding constants of 7.5±2.3 and 8.9±1.4 μM for PAP2 and MYBL2, respectively. This implies that MYBL2 will not inhibit a MBW complex containing GL3 as easily as for a complex containing EGL3. In transgenic plants where EGL3 reaches high concentrations compared with MYBL2 the equilibrium is shifted and MYBL2 is not likely to be an efficient competitor, hence anthocyanin formation could be restored by either EGL3 or GL3 genes when overexpressed by help of the 35S promoter. The present work underpins that GL3 is essential for anthocyanin accumulation under nitrogen depletion not only due to transcriptional activation, but also because of binding properties to proteins promoting or inhibiting the activity of the MBW complex.

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