Control of katanin and p53 functions by modification with ubiquitin and ISG15

Abstract

Katanin is a heterodimeric enzyme that severs and disassembles microtubules. While the p60 subunit has the enzyme activity, the p80 subunit regulates the p60 activity. The microtubule-severing activity of katanin plays an essential role in axonal growth. However, the mechanisms how neuronal cells regulate the expression of katanin-p60 remained unknown. Here I showed that USP47 and CHIP antagonistically regulate the stability of katanin-p60 and thereby axonal growth. USP47 was identified as a katanin-p60-specific deubiquitinating enzyme for its stabilization. I also identified CHIP as an ubiquitin E3 ligase that promotes proteasome-mediated degradation of katanin-p60. Moreover, USP47 promoted axonal growth of cultured rat hippocampal neurons, whereas CHIP inhibited it. Significantly, treatment with basic fibroblast growth factor (bFGF), an inducer of axonal growth, increased the levels of USP47 and katanin-p60, but not CHIP. Consistently, bFGF treatment resulted in a marked decrease in the level of ubiquitinated katanin-p60 and thereby in promotion of axonal growth. On the other hand, the level of USP47, but not CHIP, decreased concurrently with that of katanin-p60 as axons reach their target cells. These results indicate that USP47 plays a crucial role in the control of axonal growth during neuronal development by antagonizing CHIP-mediated katanin-p60 degradation. Interferon-stimulated gene 15 (ISG15), was first identified as an inducing protein by type-I IFN, lipopolysaccharide, and viruses. However, it has also been suggested to serve as a tumor suppressor, based on the recent studies that cancer chemotherapeutic drugs, such as doxorubicin and camptothecin, increase the levels of ISG15 and its conjugates. Moreover, the tumor suppressor p53 has been identified as a target protein for ISGylation. However, how p53 ISGylation is induced and how it controls tumorigenesis remain unknown. Here I showed that ISGylation of p53 plays a crucial role in the control of its apoptotic function. Genotoxic stress, such as treatment with doxorubicin, camptothecin and UV, markedly induced p53 ISGylation. In addition, the ISG15 acceptor site was identified as Lys292 by deletion analysis. Moreover, estrogen responsive finger protein (EFP) was identified as a p53-specific ISG15 E3 ligase. Intriguingly, doxorubicin-mediated increase in p53 ISGlyation led to a significant enhancement of p53 transactivity, resulting in increased expression of its target genes, such as p21 and BAX. These findings indicate that p53 ISGylation plays a critical role in the control of its function and in turn tumor suppression.