SP-RING finger found in the Siz/PIAS RING (SP-RING) family of SUMO E3 ligases
The SP-RING family includes PIAS (protein inhibitor of activated STAT) proteins, Zmiz proteins, and Siz proteins from plants and fungi. 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. PIAS proteins have SUMO E3-ligase activity and interaction of PIAS proteins with transcription factors often results in sumoylation of that protein. Zmiz1 (Zimp10) and its homolog Zmiz2 (Zimp7) were initially identified in humans as androgen receptor (AR) interacting proteins and function as transcriptional co-activators. They interact with BRG1, the catalytic subunit of the SWI-SNF remodeling complex. They also associate with other hormone nuclear receptors and transcription factors such as p53 and Smad3/Smad4, and regulate transcription of specific target genes by altering their chromatin structure. SIZ1 proteins from plants and fungi are also founding members of this family. SIZ1-mediated conjugation of SUMO1 and SUMO2 to other intracellular proteins is essential in Arabidopsis. Yeast SIZ proteins are SUMO E3 ligases involved in a novel pathway of chromosome maintenance. They enhance SUMO modification to many substrates in vivo, but also exhibit unique substrate specificity. All family members contain 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. 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.