Enables ubiquitin-ubiquitin ligase activity. Acts upstream of or within protein polyubiquitination. Located in cytoplasm. Is expressed in ganglia; medulla oblongata basal plate; thymus primordium; ventral grey horn; and vibrissa. Orthologous to human UBE4A (ubiquitination factor E4A). [provided by Alliance of Genome Resources, Apr 2022]