SP-RING finger found in protein inhibitor of activated signal transducer and activator of transcription (PIAS) proteins
The PIAS (protein inhibitor of activated STAT) protein family modulates the activity of several transcription factors and acts as an E3 ubiquitin ligase in the sumoylation pathway. It consists of four members: PIAS1, PIAS2 (also known as PIASx), PIAS3, and PIAS4 (also known as PIASy). PIAS proteins were initially identified as inhibitors of activated STAT only, but are now known to interact with and modulate several other proteins, including androgen receptor (AR), tumor suppressor p53, and the transforming growth factor-beta (TGF-beta) signaling protein SMAD. They interact with STATs in a cytokine-dependent manner. PIAS1, PIAS2, and PIAS3 interact with STAT1, STAT3, and STAT4, respectively. In addition, PIAS4 is associated with STAT1. PIAS proteins have SUMO E3-ligase activity and interaction of PIAS proteins with transcription factors often results in sumoylation of that protein. PIAS proteins contain an N-terminal SAP (scaffold attachment factor A/B (SAF-A/B), acinus and PIAS) box with the LXXLL signature, which is required for the trans-repression of STAT1 activity by PIAS2, a PINT motif, which is essential for nuclear retention of PIAS3L (the long form of PIAS3), a specific RING finger known as Siz/PIAS (protein inhibitor of activated signal transducer and activator of transcription) RING (SP-RING) finger, which is essential for SUMO ligase activity, and the acidic C-terminal domain, which is involved in binding of PIAS3 to the nuclear coactivator TIF2. The SP-RING finger is a variant of RING finger, which lacks the second, fifth, and sixth zinc-binding residues of the consensus C3H2C3-/C3HC4-type RING fingers.