Methods of identifying agents which alter the NAD-dependent acetylation status and mono-ADP-ribosylation of nuclear proteins are disclosed. The methods further include identifying agents which alter the life span or aging of a cell or an organism by determining the level of NAD-dependent acetylation and/or ADP ribosylation of a nuclear protein. The invention also relates to a mammalian Sir2 protein which acetylates or deacetylates nuclear proteins in a NAD-dependent manner and has mono-ADP-ribosyltransferase activity. Host cells producing the Sir2 protein and antibodies to the Sir2 protein are also provided.