Part 2- Hoty HDzip
In the past decade, significant progress has been made in understanding the functional significance of the biochemical properties of HD-ZIP IV START domains. First, the START domains of PDF2, ATML1, and GLABRA2 (GL2) bind to lipid ligands to stimulate TF activity in yeast cells (Schrick et al. 2014). Second, the START domains of ATML1 and GL2 bind to their lipid ligands to protect them from turnover and affect the level of dimeric proteins in plant cells (Mukherjee et al. 2022; Nagata and Abe 2021; Nagata et al. 2021). In particular, very-long-chain fatty acid-containing ceramides (VLCFA-Cers), which are lipid ligands in the START domain of PDF2 and ATML1, are thought to be generated predominantly in the outermost cells (Nagata et al. 2021). Consequently, the interaction of the START domain with VLCFA-Cer protects ATML1 from turnover in the outermost cells and increases the level of dimeric ATML1 in the epidermis. Thus, through the lipid-binding properties of the START domain, ATML1 plays a key role in the differentiation of epidermal cells at a precise location.
Part 2- Hoty HDzip
The molecular mechanism that regulates the turnover of PDF2 and ATML1 as well as affects the levels of dimeric PDF2 and ATML1, in a VLCFA-Cer-dependent manner, still needs to be addressed in future studies. In particular, it remains unclear whether the negative effects of the W463L mutation in PDF2 and the W471L mutation in ATML1 on the levels of dimeric PDF2 and ATML1 represent direct consequences of the compromised activity of their START domains. Our transient expression assay using Nicotiana benthamiana leaves established that protein accumulation was negatively affected by the introduction of the W463L mutation into PDF2 and the W471L mutation into ATML1. Thus, the negative effect of the W463L mutation in PDF2 and the W471L mutation in ATML1 on the levels of dimeric PDF2 and ATML1 may be attributed to the reduced accumulation of PDF2 and ATML1 in yeast and plant cells. The homodimerization of PHABULOSA (PHB), which belongs to the HD-ZIP III TF family, has been shown to be directly facilitated by its START domain activity (Husbands et al. 2022). Although extensive evolutionary divergence exists between HD-ZIP III and HD-ZIP IV TFs, it remains possible that the dimerization of PDF2 and ATML1 may be directly regulated by the activity of the START domain, as in PHB.
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Ruben Lerma-Reyes, Ph.D. Student, Interdepartmental Genetics Program. B.S. 2017, Newman University. Ruben is the founder and formerly served as the President of the Newman University Gardening Club. He was the recipient of a Conviron ASPB Scholar Award for 2018-19. Project: Functional Characterization of HD-Zip IV Transcription factors in Arabidopsis and Penium margaritaceum. Ruben moved to the laboratory of Brad Olson in Fall 2020. 041b061a72