The herpes simplex virus (HSV) tegument protein VP1-2 is essential for virus entry and assembly. VP1-2 also contains a highly conserved ubiquitin specific protease (USP) domain within its N-terminal region. Despite conservation of the USP and the demonstration that it can act on artificial substrates such as polyubiquitin chains, identification of the relevance of the USP in vivo to levels or function of any substrate remains limited. Here we show that HSV VP1-2 USP can act on itself and is important for stability. VP1-2 N-terminal variants encompassing the core USP domain itself were not affected by mutation of the catalytic cysteine residue (C65). However extending the N-terminal region resulted in protein species requiring USP activity for accumulation. In this context C65A mutation resulted in a drastic reduction in protein levels which could be stabilised by proteosomal inhibition, or by the presence of normal C65. The functional USP domain could increase abundance of unstable variants, indicating action at least in part, in trans. Interestingly full length variants containing the inactive USP, although unstable when expressed in isolation, were stabilised by virus infection. The catalytically inactive VP1-2 retained complementation activity of a VP1-2 negative virus. Furthermore a recombinant virus expressing a C65A mutant VP1-2 exhibited little difference in single step growth curves and the kinetics and abundance of VP1-2 or a number of test proteins. Despite the absence of a phenotype for these replication parameters, the USP activity of VP1-2 may be required for function, including its own stability, under certain circumstances.