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CUL4B
Cullin-4B is a protein that in humans is encoded by the ''CUL4B'' gene which is located on the X chromosome. CUL4B has high sequence similarity with CUL4A, with which it shares certain E3 ubiquitin ligase functions. CUL4B is largely expressed in the nucleus and regulates several key functions including: cell cycle progression, chromatin remodeling and neurological and placental development in mice. In humans, ''CUL4B'' has been implicated in X-linked intellectual disability and is frequently mutated in pancreatic adenocarcinomas and a small percentage of various lung cancers. Viruses such as HIV can also co-opt CUL4B-based complexes to promote viral pathogenesis. CUL4B complexes containing Cereblon are also targeted by the teratogenic drug thalidomide.
== Structure ==
Human CUL4B is 913 amino acids long and shares a high degree of sequence identity (84%) with CUL4A with the exception of its unique N-terminal region. The extreme N-terminus of CUL4B is disordered and, currently, it is unclear what structural and functional qualities it possesses. CUL4B binds to the beta-propeller of the DDB1 adaptor protein which interacts with numerous DDB1-CUL4-Associated Factors (DCAFs). This interaction is crucial for the recruitment of substrates to the ubiquitin ligase complex. At the C-terminal end, CUL4B interacts with the RBX1/ROC1 protein via its RING domain. RBX1 is a core component of Cullin-RING ubiquitin ligase (CRL) complexes and functions to recruit E2 ubiquitin conjugating enzymes. Therefore, the C-terminus of CUL4B - along with RBX1 and activated E2 enzymes - compose the catalytic core of CRL4B complexes. CUL4B is also modified by covalent attachment of a NEDD8 molecule at a highly conserved lysine residue in the C-terminal region. This modification appears to induce conformational changes which promotes flexibility in the RING domain of cullin proteins and enhanced ubiquitin ligase activity.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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