A Structure for Plant-Specific Transcription Factors: The GRAS Domain Revealed.

The GRAS protein family is illustrious. The GRAS domain is plant specific, named for the first three proteins found to contain it: GIBBERELLIC ACID INSENSITIVE (GAI), REPRESSOR of GAI, and SCARECROW. In addition to these foundingmembers,which function in gibberellin signaling and root patterning, the GRAS family includes DELLA proteins—important in gibberellin, jasmonate, and light signaling—as well as Nodulation Signaling Pathway proteins, which regulate nodulation in legumes (reviewed in Sun et al., 2012). GRAS family proteins have been reported in almost 300 plant species and generally appear to influence plant growth, development, and responses to environment via transcriptional regulation. Whereas their N-terminal regions vary, members of this family are characterized by a C-terminal region harboring the GRAS domain. In a landmark article, Li et al. (2016) report the crystal structure of the GRAS domain, thereby providing long-awaited insight into potential mechanisms underlying the widespread importance of GRAS domain proteins. Li et al. crystalized the GRAS domain of rice (Oryza sativa) SCARECROW-LIKE7 (SCL7). The crystal structure showed that the domain forms a core region topped with a cap structure (see figure). GRAS domains comprise five conserved subdomains: LRI, VHIID, LRII, PFYRE, and SAW. The cap of the GRAS domain structure was formed of a-helices of the LRI subdomain with two from the PFYRE subdomain. The remaining subdomains made up the core of the structure. The region between the helices of the LRI cap and the VHIID core was missing, suggesting that there could bemovement in this area. Interestingly, the core structure of the GRAS domain consisted of a b-sheet of sandwichedbetweena-helices, resembling a Rossmann fold. This finding lends support to the idea that GRAS proteins should be considered part of the Rossmann fold superfamily of methyltransferases despite lacking residues for methyltransferase activity (Zhang et al., 2012). The Os-SCL7 GRAS domain structure provides important insights into the potential roles of the various GRAS subdomains. The LRI subdomain contains a characteristic leucine-rich repeat, which formed a leucine zipper in the structure, and a nuclear localization signal, which was placed on the surface in the structure. In the VHIID subdomain, the conserved IHIVD motif formed a b-strand that appeared to be important for stabilizing the overall structure of the GRAS domain. The leucine repeats of the LRII subdomain were positioned so that theymight interact with other proteins. Both thePFYREandSAWsubdomains appeared tocontribute to themaintaining thestructure of the domain. Importantly, Os-SCL7 was crystallized as a dimer, with a groove formed between Crystal structure of the GRAS domain. The five subdomains of the Os-SCL7 GRAS domain are colored according to the schematic below the structure (adapted from Figure 1 of Li et al. [2016] ).