Structural domains comprising this superfamily share the structure of the PINIT domain, found in the Siz/PIAS RING family of SUMO E3 ligases. This proteins present a tripartite architecture with an N-terminal PINIT domain, a central zinc-containing RING-like SP-RING domain, and a C-terminal domain desginated SP-CTD. Structural and biochemical studies have shown that both SP-RING and SP-CTD are required for activation of the E2-SUMO thioester while the PINIT domain is required for redirecting SUMO conjugation to the proliferating cell nuclear antigen (PCNA) at lysine 164, a non-consensus lysine residue that is not modified by the SUMO E2 in the absence of Siz1 PMID:19748360.