Os02g0244100

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Annotated Information

Function

grain phenotype and structure of GW2 (from reference).

Characterization of grain milk filling in FAZ1 and NIL(GW2) and proposed model for the role of GW2. (a) Time-course of endosperm fresh weight increase. (b) Time-course of endosperm dry weight increase. Data are mean ± s.d. (n ¼ B10 to B15 plants) in a and b. (c) Proposed model for the role of GW2 in regulation of grain width (size) and weight. GW2 recruits the targeted substrate for degradation, thereby inhibiting cell division, and then influences spikelet hull size, subsequently indirectly influencing milk filling rate, endosperm cell size, endosperm size and, ultimately, grain size (width, weight and yield) (from reference).

This gene is reported as GW2, a new QTL that controls rice grain width and weight. GW2 encodes a previously unknown RING-type protein with E3 ubiquitin ligase activity, which is known to function in the degradation by the ubiquitin-proteasome pathway. Loss of GW2 function increased cell numbers, resulting in a larger (wider) spikelet hull, and it accelerated the grain milk filling rate, resulting in enhanced grain width, weight and yield. GW2 negatively regulates cell division by targeting its substrate(s) to proteasomes for regulated proteolysis. The functional characterization of GW2 provides insight into the mechanism of seed development and is a potential tool for improving grain yield in crops. Reduced expression of GW2 increases grain size (mainly grain width), resulting in enhanced grain weight, whereas overexpression decreases grain size and weight(from reference).

The naturally occurring WY3 allele of GW2, which encodes a truncated version of the protein with a 310–amino acid deletion, increases the number of cells of the spikelet hull, resulting in a wider spikelet hull, and subsequently accelerates the grain milk filling rate, resulting in increased grain width, weight and yield. Many RING-type proteins function as E3 ubiquitin-protein ligases, targeting proteins for ubiquitin-dependent degradation by the 26S proteasome28–31,38,39. Such RING proteins are involved in the regulation of numerous cellular processes, including transcription, signal transduction, recombination and cell cycle progression. A previous study has shown that the RING-type (C3H2C3) protein BIG BROTHER, which has E3 ligase activity, acts as a central negative regulator of A. thaliana floral organ size, most likely by marking cellular proteins for degradation39. In contrast, the GW2, a new RING-type protein, has E3 ubiquitin ligase activity and alters the number of cells in the spikelet hull, suggesting that GW2 E3 ligase functions as a regulator of cell division through ubiquitin-mediated proteolysis. However, the mechanism of cell cycle regulation mediated by GW2 remains to be elucidated. In the majority of RING-type proteins, the N terminus contains the RING domain that binds E2, and the remainder contains other protein–protein interaction domains that may function as the substrate-binding domain of the E3 ligase24. For example, COP1 has both a RING-type domain in the N terminus and a WD-40 repeat domain in the C terminus that can bind several protein targets, including HY5, thereby recruiting an E2 and targeting HY5 and other substrates for ubiquitination and degradation by the proteasome40. In addition, the WY3 variant of GW2 has an intact RING domain, thereby retaining E3 ubiquitin ligase activity, but is truncated by 310 amino acids that might contain the substrate-binding domain. The absence of a substrate-binding domain suggests that WY3 GW2 is a null allele. The coincidence of the GW2 null allele and the increased number of cells in the spikelet hull suggest that WY3 GW2 protein does not interact with the substrate(s) involved in cell division and does not target them for ubiquitination and subsequent degradation. These data suggest that GW2 E3 ligase is a new negative regulator of cell division, targeting its substrate(s) to proteasomes for regulated proteolysis. We examined GW2 for the presence of other known domains but did not find any known domains in GW2. Identification and characterization of both the substrate-binding domain of GW2 and the GW2 target substrate will be challenging but worth pursuing(from reference).

Expression

Cellular properties and expression pattern of GW2. (from reference).

Transient expression in onion epidermal cells showed that GFP-GW2 localized to the cytoplasm.RT-PCR data in both FAZ1 and NIL(GW2) showed that GW2 mRNA was expressed constitutively in shoots and roots of seedlings, inflorescent meristems, young flowers, leaves and spikelet hulls and endosperms 4 d after fertilization.The GW2 promoter–GFP expression analysis showed that GFP was strongly expressed in roots , leaves, and floral organs including stamens, pistils and hulls.

GW2 increases grain size and yield in rice, which is a new RING-type protein with intrinsic E3 ubiquitin ligase activity that localizes to the cytoplasm and is constitutively expressed in various tissues. Reduced expression of GW2 increases grain size (mainly grain width), resulting in enhanced grain weight, whereas overexpression decreases grain size and weight. The naturally occurring WY3 allele of GW2, which encodes a truncated version of the protein with a 310–amino acid deletion, increases the number of cells of the spikelet hull, resulting in a wider spikelet hull, and subsequently accelerates the grain milk filling rate, resulting in increased grain width, weight and yield.

Evolution

A.Relationship of ZmGW2-CHR4 and ZmGW2-CHR5. B.Phylogenetic tree of GW2 protein sequences in maize, rice, sorghum and barley. (from reference).

The rice GW2 gene has two co-orthologs located on duplicated chromosomes in maize. Both genes consist of eight exons, with an overall sequence similarity of 94% to each other and 93% to the rice GW2 gene across the coding region. They were named ZmGW2-CHR4 and ZmGW2-CHR5.

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Labs working on this gene

1National Key Laboratory of Plant Molecular Genetics, Shanghai Institute of Plant Physiology and Ecology, Shanghai Institute for Biological Sciences, The Chinese Academy of Sciences, 300 Fenglin Road, Shanghai 200032, China.