Post-replicational modification

Moreover, overexpression of the same mutants inhibit DNA replication and block the cells at the Gl/S-phase transition (Kim et al, 2005), emphasizing the potential role of nucleolin mobilization. It is therefore highly probable that two different processes help the formation of RPA-nucleolin complexes after a genotoxic stress a post-transcriptional modification of nucleolin that renders the GAR domain of nucleolin accessible to RPA, and its p53-dependent relocalization to the nucleoplasm where a higher amount of RPA is available. Of importance, nucleolin relocalization is transient and lasts far less than replication inhibition (Daniely and Borowiec, 2000). This means that nucleolin-RPA interaction is only an initial event and that other mechanisms account for prolonged replication inhibition. 

In contrast, lysine methylation seems to be an exceptionally stable modification. Early studies showed that turnover of histone methyl groups was even slower than turnover of the histones themselves (e.g., [26,27]). No conclusive evidence has yet been found for histone demethylating enzymes, and they may not exist [28]. It may be that removal of methylated histones mostly occurs passively, through post-replication chromatin assembly and replacement of old, methylated histones with new, unmethylated ones. However, the possibility remains that local methylation patterns may be more dynamic and may involve novel mechanisms for removal of methylated tails [28]. 

The extreme level of BF inhibition is in contrast to steady-state analysis of human Pol 8. The degree of inhibition by 06-MeG observed for both the replicative Pol 8 (with the sliding clamp) and three recombinant Y-family polymerases (T, I, and k) was found to be relatively modest (10- to 100-fold) [149], With one exception, all of the human enzymes tested in the aforementioned study incorporated dCTP and dTTP equally well opposite 06-MeG. The exception to this pattern was human Pol t, which performed insertion of dTTP opposite 06-MeG 10 times better than dCTP opposite the lesion and with greater efficiency than dCTP opposite G (around 2-fold). It is entirely possible that inclusion of accessory proteins and/or post-translational modifications to the Y-family members could influence the catalytic efficiency of (/ -MeG bypass. In vitro studies with all polymerases studied to date verify the mutagenic potential of the 06-MeG adduct. 

Some viral proteins with co/post-translational modifications, such as myristoylation and phosphorylation, also support HIV-1 replication. V-myristoylation of pi7 of HIV-1 is essential for structural assembly and replication (Bryant and Ratner, 1990, Shiraishi et al, 2001, Tashiro et al, 1989). Phosphorylation of pi7 is related to its dissociation from the membrane during the early post-entry step of HIV-1 (Bukrinskaya et al, 1996, Gallay et al, 1995). These results suggest that the identification of new cellular proteins and co/post-translational modifications of viral and cellular proteins that are indispensable to viral replication is needed to accomplish future AIDS-therapeutic breakthroughs. 

Figure 1. Strategy for the identification of proteins and its co/post-translational modifications that are indispensable for HfV-1 replication.

Proteome analysis of HIV-1lav-i is veiy useful in understanding biological phenomena at the molecular level by identifying new proteins and co/post-translational modifications that are indispensable to viral replication, and in finding out attractive targets for the future AIDS-therapies. 

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