E3 ubiquitin-protein ligase involved in the regulation of grain size. May limit grain width and weight by restricting cell proliferation of the spikelet hull. Possesses E3 ubiquitin-protein ligase activity in vitro. S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Expressed in roots, shoots, leaves, inflorescence meristems, stamens, pistils, spikelet hulls and endosperms 4 days after fertilization. Plants silencing GW2 produce grains with increased width, resulting in enhanced grain weight, whereas overexpression of GW2 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.