Cullin
structure of the cul1-rbx1-skp1-f boxskp2 scf ubiquitin ligase complex
Identifiers
Symbol	Cullin
Pfam	PF00888
InterPro	IPR001373
PROSITE	PDOC00967
SCOP2	1ldj / SCOPe / SUPFAM
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Cullin protein neddylation domain
structure of the cul1-rbx1-skp1-f boxskp2 scf ubiquitin ligase complex
Identifiers
Symbol	Cullin_Nedd8
Pfam	PF10557
InterPro	IPR019559
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Cullins are a family of hydrophobic scaffold proteins which provide support for ubiquitin ligases (E3). All eukaryotes appear to have cullins. They combine with RING proteins to form Cullin-RING ubiquitin ligases (CRLs) that are highly diverse and play a role in myriad cellular processes, most notably protein degradation by ubiquitination.^[1][2]

The human genome contains eight cullin genes

• CUL1, part of SCF complex
• CUL2, part of ECS complex (Elongin C - CUL2 - SOCS-box)
• CUL3, part of CUL3-BTB complex
• CUL4A
• CUL4B
• CUL5
• CUL7
• CUL9, also known as PARC

There is also a more distant member called ANAPC2 (or APC2), part of the Anaphase-promoting complex.

CUL1, 2, 3, 4A, 4B, 5 and 7 each form part of a multi-subunit ubiquitin complex.

Cullin-RING ubiquitin ligases

Cullin-RING ubiquitin ligases (CRLs), such as Cul1 (SCF) play an essential role in targeting proteins for ubiquitin-mediated destruction; as such, they are diverse in terms of composition and function, regulating many different processes from glucose sensing and DNA replication to limb patterning and circadian rhythms.^[3] The catalytic core of CRLs consists of a RING protein and a cullin family member. For Cul1, the C-terminal cullin-homology domain binds the RING protein. The RING protein appears to function as a docking site for ubiquitin-conjugating enzymes (E2s). Other proteins contain a cullin-homology domain, such as CUL9, also known as p53 cytoplasmic anchor PARC, and the ANAPC2 subunit of the anaphase-promoting complex/cyclosome; both CUL9 and ANAPC2 have ubiquitin ligase activity. The N-terminal region of cullins is more variable, and is used to interact with specific adaptor proteins.^[4][5][6]

Modification by NEDD8

With the exception of ANAPC2, each member of the cullin family is modified by Nedd8 and several cullins function in Ubiquitin-dependent proteolysis, a process in which the 26S proteasome recognises and subsequently degrades a target protein tagged with K48-linked poly-ubiquitin chains. Nedd8/Rub1 is a small ubiquitin-like protein, which was originally found to be conjugated to Cdc53, a cullin component of the SCF (Skp1-Cdc53/CUL1-F-box protein) E3 Ub ligase complex in Saccharomyces cerevisiae (Baker's yeast), and Nedd8 modification has now emerged as a regulatory pathway of fundamental importance for cell cycle control and for embryogenesis in metazoans. The only identified Nedd8 substrates are cullins. Neddylation results in covalent conjugation of a Nedd8 moiety onto a conserved cullin lysine residue.^[7]