期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：9
DOI：10.1073/pnas.2117774119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Whether animal neurons or plant cellular valves (called stomata), specialized cell-type differentiation is directed by the lineage-specific basic helix–loop–helix (bHLH) transcription factors that typically form heterodimers with ubiquitous bHLH proteins. How does a broadly expressed bHLH protein switch its lineage-specific heterodimeric partners? Here we identify a structural module, called the ACT-like domain, in the plant bHLH protein SCREAM. This domain plays a role in partner bHLH selectivity and is critical for the proliferation-to-differentiation switch within the cell lineages to make stomata, plant cellular valves for gas exchange and water control. Our work provides mechanistic insight into how plant transcription factors control cell-fate specification through an unanticipated heterodimeric partner selectivity interface.
Multicellular organisms develop specialized cell types to achieve complex functions of tissues and organs. The basic helix–loop–helix (bHLH) proteins act as master regulatory transcription factors of such specialized cell types. Plant stomata are cellular valves in the aerial epidermis for efficient gas exchange and water control. Stomatal differentiation is governed by sequential actions of three lineage-specific bHLH proteins, SPEECHLESS (SPCH), MUTE, and FAMA, specifying initiation and proliferation, commitment, and terminal differentiation, respectively. A broadly expressed bHLH, SCREAM (SCRM), heterodimerizes with SPCH/MUTE/FAMA and drives stomatal differentiation via switching its partners. Yet nothing is known about its heterodimerization properties or partner preference. Here, we report the role of the SCRM C-terminal ACT-like (ACTL) domain for heterodimerization selectivity. Our intragenic suppressor screen of a dominant
scrm-D mutant identified the ACTL domain as a mutation hotspot. Removal of this domain or loss of its structural integrity abolishes heterodimerization with MUTE, but not with SPCH or FAMA, and selectively abrogates the MUTE direct target gene expression. Consequently, the
scrm-D ACTL mutants confer massive clusters of arrested stomatal precursor cells that cannot commit to differentiation when redundancy is removed. Structural and biophysical studies further show that SPCH, MUTE, and FAMA also possess the C-terminal ACTL domain, and that ACTL•ACTL heterodimerization is sufficient for partner selectivity. Our work elucidates a role for the SCRM ACTL domain in the MUTE-governed proliferation–differentiation switch and suggests mechanistic insight into the biological function of the ACTL domain, a module uniquely associated with plant bHLH proteins, as a heterodimeric partner selectivity interface.
