sentence: Importantly , the function of Ubp10 as ubPCNA ubiquitin-specific protease is separable from histone H2B ubiquitylation , as Ubp10 deubiquitylates ubPCNA in cells lacking Bre1 , the E3 ubiquitin ligase that in complex with Rad6 monoubiquitylates histone H2B [ 31 ] , [ 51 ] .