CUL9 (also known as PARC for p53-associated PARkin-like cytoplasmic protein) is an atypical cullin that has been shown to form a ubiquitin ligase complex with RBX1, although other components of the putative CRL9 complex have not yet been identified (Skaar et al, 2007; Li et al, 2014). CUL9:RBX1 is neddylated in vivo, likely through the UBE2M E2 although this hasn't been directly demonstrated (Skaar et al, 2007).
CUL9 is 60% identical to CUL7, another atypical mammalian cullin family member, but more distantly related to CUL1, 2, 3, 4A,4B and 5. CUL9 and CUL7 have been shown to form a heterodimer in vivo, and both interact with p53 (Skaar et al, 2007; Andrews et al, 2006; Nikolaev et al, 2003). CUL9 ubiquitinates BIRC5 (also known as Survivin), a protein with roles in cellular proliferation and inhibition of apoptosis. CUL9-mediated ubiquitination of BIRC5 is inhibited by the 3M complex, which consists of CUL7, CCDC8 and OBSL1 (Li et al, 2014).