Recognizes and hydrolyzes the peptide bond at the C-terminal Gly of ubiquitin. Involved in the processing of poly-ubiquitin precursors as well as that of ubiquitinated proteins (Probable). Involved in the regulation of grain size (PubMed:30796160). Acts as positive regulator of grain width and size by influencing cell proliferation (PubMed:30796160). Functions partially antagonistically with GW2 in the regulation of grain width (PubMed:30796160). Possesses deubiquitinating enzyme activity in vitro (PubMed:30796160). Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). Highly expressed in young panicles (PubMed:30796160). Expressed in roots, leaf blades, leaf sheaths and stems (PubMed:30796160). Expressed at low levels in brown grains (PubMed:30796160). The dominant gain-of-function large grain1-D (lg1-D) develops significantly enlarged grains. Belongs to the peptidase C19 family. Truncated N-terminus.