Full-length GLI proteins are labile transcription factors that are rapidly degraded after ubiquitination by the SPOP:CUL3:RBX1 E3 ligase (Ohlmeyer et al, 1998; Humke et al, 2010; Tukachinsky et al, 2010; Chen et al, 2009; Zhang et al, 2009; Wen et al, 2010). SPOP (Speckle-type POZ protein) is the vertebrate homologue of Drosophila Hib/Roadkill, which was identified as a negative regulator of Hh signaling (Ohlmeyer et al, 1998; Zhang et al, 2006; Kent et al, 2006). SPOP/Hib proteins contain BTB and MATH domains and function as the substrate-binding component of the E3 ligase complex, where they promote oligomerization (Zhang et al, 2009; Furukawa et al, 2003; Zhuang et al, 2009). SPOP has been shown to bind to GLI2 and GLI3 through multiple serine and threonine rich motifs in the transcription factors, but direct binding to GLI1 has not been demonstrated (Cheng et al, 2009; Zhang et al, 2009).
The stability of SPOP itself is regulated in an unknown manner by DZIP1, a regulator of Hh signaling best characterized in zebrafish for its positive role in promoting ciliogenesis (Sekimizu et al, 2004; Wolff et al, 2004; Glazer et al, 2010; Kim et al, 2010; Tay et al, 2010; Wang et al, 2013). More recently, DZIP1 has also been shown to act as a negative regulator of Hh signaling by preventing the ubiquitin- and proteasome-dependent degradation of SPOP, and in this way increasing the turnover of activated GLI proteins (Jin et al, 2011; Schwend et al, 2013).