Cullin 4A

The CUL4A protein has a length of 759 amino acids and forms an expanded, rigid structure consisting primarily of alpha helices . The N-terminus of CUL4A binds to the , which interacts with numerous factors associated with DDB1-CUL4 (DCAF (s)). As a result, the N-terminus is crucial for the recruitment of substrates for the ubiquitin ligase complex. At the end of the C-terminus, CUL4A interacts with the / ROC1 protein through its . RBX1 is the main component of the Cullin-RING ubiquitin ligase (CRL) complex and recruits . Thus, the C-terminus of CUL4A — together with RBX1 and activated E2 enzymes — is the catalytic core of the CRL4 complex. CUL4A is also modified by covalent attachment of the molecule to the highly conserved lysine residue in the C-terminal region. This modification, apparently, causes conformational changes that stimulate flexibility in the RING domain of cullin proteins and increased ubiquitin ligase activity ^[3] .

In general, CRL4A complexes have a modular structure that allows complex cell regulation and the effect on numerous substrates and processes in the cell. Although the individual parts vary, all the main kullin ubiquitin ligases possess these properties ^[4] .

DNA damage and repair

DDB1 was originally characterized as a large subunit of the heterodimeric complex (UV-DDB), which serves to recognize damaged DNA and participate in the form of repair, known as excision repair of nucleotides (NER). The smaller subunit of this DNA damage binding protein complex is known as and is able to directly bind DNA damage associated with UV radiation. DDB2 is a DCAF protein and simultaneously a ubiquitinated substrate of the CRL4 complex, and also serves as an E3 protein ligase for other substrates, such as and histones (see the next section) near the site of damage ^[5] . Due to its ubiquitination of DNA damage and its ability to recognize DDB2 and XPC proteins, CUL4A has been described as a negative regulator of NER activity ^{[6] [7]} . In addition to the “global” type of NER, the CRL4A complex also seems to play the role of a “transcriptional pair” of NER in conjunction with the protein ^[8] . CRL4A complexes were apparently activated by certain types of DNA damage (especially UV irradiation) and some substrates were ubiquitinated after the induction of DNA damage.

Chromatin Remodeling

The role of CUL4A in chromatin modification is largely related to DNA repair activity and occurs after the induction of DNA damage. Both CUL4A and its closely related homolog CUL4B can ubiquitinate histones H2A, H3, and H4 ^{[9] [10]} . The yeast homologues CUL4A, Rtt101 ubiquitinate H3 histones and facilitate nucleosome assembly; the CRL4A complex performs similar functions in human cells ^[11] . CRL4 complexes also affect histone methylation events and chromatin structures by regulating ^[12] . Histone ubiquitinated by the CRL4 chromatin complex (Cdt2) in the S phase and subsequent DNA damage to the -dependent manner ^{[13] [14] [15]} .

Cell Cycle Regulation and DNA Replication

CRL4A complexes regulate entry into the phase of DNA synthesis, or the S phase of the mitotic cycle, by regulating expression levels of the licensing factor of the replication factor protein and the cyclin-dependent kinase inhibitor p21 . In both cases, CRL4A uses as DCAF, linking both substrates in a PCNA-dependent manner. In the undisturbed progression of the cell cycle , ubiquitination and inhibition of these proteins by the CRL4A ^Cdt2 complex occurs at the beginning of DNA replication. DNA damage, such as due to UV irradiation, also induces CRL4A ^Cdt2 to indirectly kill these proteins. Interestingly, both substrates are also regulated by the CRL4A ^Cdt2 complex .

CRL4-mediated destruction of p21 removes the inhibition of - CdK2 and stimulates entry into the S phase. Loss of Cdt2 expression increases p21 expression in cells and stabilizes p21 after UV irradiation ^[16] . CUL4A deletes the results of delayed entry into the S-phase in mouse embryonic fibroblasts , which were saved from the removal of p21 ^[7] .

After stimulation of the initiation of eukaryotic DNA replication at the origin of replication , Cdt1 is inactivated by and targeted degradation by the SCF complexes ^Skp2 and CRL4 ^Cdt2 . The expression of Cdt1 is stabilized by the RNA interference-mediated impact of DDB1 or both, both CUL4A and CUL4B, which suggests the excessive or overlapping functions of both CUL4 proteins to regulate Cdt1 ^{[17] [18]} . Only a decrease in geminin expression ( Geminin ) seems to cause replication in overexpressing Cdt1 cells.

Hematopoiesis

CRL4A complexes appear to induce the degradation of numerous members of the transcriptional family of HOX , which are necessary regulators of hematopoiesis ^[19] . The first member of the HOX family, which was defined as the target of CRL4A-mediated degradation, is HOXA9, which is very important for maintaining stem cell hematopoiesis and was involved in the subgroup of ^{[20] [21]} . HOXA9 Degron is located within the homeodomain , which is crucial for DNA binding. Sequence alignment studies have shown that there is a highly conservative “LEXE” motif within the same helix of a homeodomain. When several amino acids within this motif mutated, HOXB4 became resistant to CRL4A-mediated degradation ^[19] . The substrate receptor or DCAF necessary for the degradation of the HOX protein remains unknown.

Spermatogenesis and meiosis

The Cul4a gene is necessary for normal spermatogenesis and meiosis in male germ cells of mice ^{[22] [23]} . Cul4a deficient males produce abnormal sperm and are infertile. Although both CUL4A and CUL4B are expressed in male gametes , CUL4A is highly expressed in pachytene and diplotene . It is at these stages that CUL4A deficiency in male germ cells shows a high level of apoptosis , improper DNA repair, and the accumulation of CRL4 substrate - .

Cancer

CUL4A is enhanced in 3 to 6% of certain carcinomas including :. breast, uterus, lungs, stomach, and colorectal cancer ^[24] . CUL4A also mutates or amplifies in about 4% of melanoma (although mutations are distributed in nature and individual mutations occur spontaneously).

In murine models, Cul4a yields an amazing result in terms of resistance to UV-induced skin carcinogenesis ^[7] . Overexpression of -induced Cul4a in mouse lung tissue contributed to hyperplasia ^[25] .

Due to the observed increase in CUL4A in several carcinomas and the fact that CRL4 complexes target several DNA and tumor suppressor gene repair, CUL4A can be considered an oncogen in some contexts.

Viral pathogenesis

Due to their strong expression (in particular, in the process of DNA replication) and the modular nature, CRL4A complexes can be co-opted or “captured” to stimulate the proliferation of the virus in mammalian cells.

Some paramyxoviruses block the response of interferon in cells by capturing STAT1 and disrupting signaling. Protein V of monkey virus 5 acts as a receptor for the substrate and the bridge of interaction between DDB1 and STAT proteins (the structure of the CRL4A ^SV5V complex in the photo on the inset), thereby causing STAT1 ubiquitination and degradation ^{[26] [27]} .

DCAF1 is also called because of its interaction with the HIV-1 . Although DCAF1 / VPRBP seems to play a crucial role in tumor suppression, DNA replication, and embryonic development, HIV-1 “captures” the ubiquitin ligase complex, inducing cell arrest in phase G2 ^{[28] [29] [30]} . CRL4A ^DCAF1-Vpr induces ubiquitination of the nuclear isoform of ^{[31] [32]} . HIV-2 also appears to use CRL4A ^DCAF1 with the protein induced by lentivirus disruption of a deoxynucleoside triphosphohydrolase inhibitor called ^{[33] [34]} .

CUL4A man directly interacts with:

• ^[35] ,
• ^{[36] [37]} ,
• ^[38] and
• Signalosome COP9 ^[39] .

The human CUL4A-DDB1-RBX1 complexes contribute to ubiquitination:

^† CRL4A substrate protein only when administered by a viral protein.

Notes

Literature