Protein-poly ation

Protein start signal Left splice site Right Protein Poly(A) Poiyadenyl-splice stop addition ation site signal signal site ... 

There is now evidence that poly(ADP-ribosyl)ation of nuclear proteins might be involved in DNA repair [1], DNA replication [2, 3] and cellular differentiation [4, 5]. A common function of nuclear protein poly(ADP-ribosyl)ation in these various events might be the alteration of chromatin structure [6,7]. 

Aubin RJ, Dam VT, Miclette J, Brousseau Y, Poirier GG (1982) Chromosomal protein poly (ADP-ribosyl)ation in pancreatic nucleosomes. Can J Biochem 60 295-305... 

Although a precise definition of the role of nuclear poly(ADP-ribosyl)ation is not available, the histone-shuttle mechanism proposed by Althaus and colleagues offers a possible unifying explanation of numerous experimental findings. While this model will come under further experimental scrutiny, the effects of ADP-ribo-sylating individual chromosomal proteins other than the polymerase itself (automodification) still needs to be elucidated. 

Assembly of the large, multiprotein cleavage/polyadenyl-ation complex around the AU-rIch poly(A) signal In a pre-mRNA is analogous In many ways to formation of the transcriptlon-prelnitlatlon complex at the AT-rIch TATA box of a template DNA molecule (see Figure 11-27). In both cases, multiprotein complexes assemble cooperatively through a network of specific protein-nucleic acid and protein-protein Interactions. 

Figure 1. Metabolism of poly(ADP-ribose). Poly(ADP-ribosyl)ation is a posttranslational modification of acceptor proteins, and poly(ADP-ribose) itself serves as a component of cell struaure. Poly(ADP-ribose) glycohydrolase (PARC) is a main enzyme to degrade poly(ADP-ribose) and forms ADP-ribose as product. Other degrading enzymes ate phosphodiesterase (or pyrophosphatase) and ADP-ribosyl protein lyase.

Previous studies had shown that PARP-1 is mainly localized to the nucleus. However from the pattern of chromosome instability in PARP-1 knockout mouse cells, we speculated that PARP-1 might also be localized to another component related to the mitotic machinery. Indeed, we found PARP-1 localized to the centrosome in some cancer cells and mouse embryonic fibroblasts (MEF). The localization of PARP-1 at the centrosome su ested the possibility that PARP-1 at the centrosome could catalyze poly(ADP-ribosyl)ation of certain centrosomal proteins. In fact we could observe the presence of various bands in western blots of the centrosomal proteins that reacted with a monoclonal antibody to polyfADP-ribose) (lOH). These results showed the involvement of PARP-1-mediated poly(ADP-ribosyl)ation in centrosome regulation. 

It is important to identify each of the poly(ADP-ribosyI)ated proteins in order to clarify the role of poly(ADP-ribosyl)ation. Indeed we have identified some proteins that are poly(ADP-ribosyl)ated in the centrosome. One of these proteins is the tumor suppressor protein p33. p33 is a well known guardian of the genome, and it is essential for DNA repair and apoptosis. Interestingly, p33 also localizes to the centrosome and r ubites centrosome fonaion directly or indirectly. Our study revealed that inhibition of poly(ADP-ribosyl)arion of p33 due to administration of a PARP inhibitor or loss of PARP-1 mi t be involved in the defect in centrosome function and chromosomal instability. 

Out data show that there are many proteins that arc poly(ADP-ribosyl)ated. In view of the dynamic process of poly(ADP-ribosyl)ation, a posttranslational modification undergoing rapid turnover, the recent discovery that poly(ADP-ribose) glycohydrolase (PARC) is also localized in the centrosome durii the cell cycle is very interesting and plausible. 

Identification of the poly(ADP-ribosyl)ated centrosomal proteins and clarification of the changes in subcellular localization by possible shutding between the centrosome and the nucleus will be essential for further understanding the role of poly(ADP-ribosyl)ation. It would also be very interesting to know the trigger(s) of poly(ADP-ribosyl)ation in the centrosome, since it is believed that this structure is devoid of DNA. Such analyses would clarify a novel mechanism of the regulation of the centrosome function by posttranslational poly(ADP-ribosyl)ation. 

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