Proliferating-Cell-Nuclear-Antigen

• OMIM: 176 740
• UniProt: P12004

Proliferating Cell Nuclear Antigen ( PCNA) is a protein that surrounds the ring (so-called clamping ring protein ) during the eukaryotic DNA replication, the DNA. Only by PCNA, it is possible that during the S phase of the cell cycle, the entire DNA can be amplified with high speed and without major disruptions.

Genetics

The PCNA gene is located on chromosome 20 in humans, are currently two known transcription variants which encode the same protein. The promoter region contains binding sites for the transcription factor E2F. Pseudogene of this gene were found on chromosome 4 and the X - chromosome.

Structure

PCNA consists of 261 amino acids and has a molecular weight of about 28.7 kDa. The protein consists of three identical subunits ( homotrimer ) forming a ring, and may bind through a total of 12 symmetrically located α -helices to double stranded DNA ( ds- DNA).

Function

In the context of eukaryotic DNA replication initially using the primase ( RNA polymerase ), short RNA primer onto the unwound synthesized template strand ( de novo synthesis). The thus formed primer-template hybrid is directly passed to the associated α DNA polymerase which extends the RNA primer at the 3 ' end and the start of DNA synthesis. Thus, the primers are made to the 5 ' end of 8-10 ribonucleotides ( RNA), and the 3'-end of 20 deoxynucleotides ( DNA). The 3'- end of the primer thus forms a portion of the ds- DNA. This transition from the ds- DNA at the primer end to single-stranded DNA (ss- DNA) of the template strand is recognized by Replication Factor C (RF -C), which touches the PCNA ring clamp. The replicative DNA polymerase δ binds to PCNA and extends the primers: - either until it reaches the next Okazaki fragment ( sequence strand) or to the next replication bubble ( leading strand ). PCNA throughout the elongation remains bound to the DNA polymerase and prevents the dissociation of the DNA strand. Only when the last deoxynucleotide is added, the polymerase falls off δ and can otherwise bind again to PCNA.

In the case of DNA damage, the tumor suppressor p53 causes the increased formation of the CDK inhibitor p21. p21 can bind among others, the PCNA ring terminal, resulting in the immediate stop of replication. This gives damaged cells time for DNA repair, thereby preventing that altered genetic material is passed on to daughter cells.

PCNA also binds to the DNA polymerase δ, if it is recruited to DNA repair. This happens especially to the single-stranded gaps in the framework of the nucleotide excision repair (NER ) arise to fill. In this case, PCNA is monosumoyliert lysine 164, whereas it supports a ubiquitin - block at the same position during replication.

The charge factor RF -C belongs to the ATPases of the AAA class.

Evolution

PCNA is functionally analogous to the beta -clamp in bacteria. The beta -clamp also binds with 12 α -helices located symmetrically to the DNA, but is - in contrast to PCNA - a homodimer. This homodimer consists of two molecules of the β subunit of DNA polymerase III. The beta -clamp is loaded by the so-called γ complex on the DNA. The γ - complex with five subunits may be considered as analogous to RF -C in the eukaryotes.