MDM2 Mediates Nonproteolytic Polyubiquitylation of the DEAD-Box RNA Helicase DDX24

Abstract

MDM2 mediates the ubiquitylation and thereby triggers the proteasomal degradation of the tumor suppressor protein p53. However, genetic evidence suggests that MDM2 contributes to multiple regulatory networks independently of p53 degradation. We have now identified the DEAD-box RNA helicase DDX24 as a nucleolar protein that interacts with MDM2. DDX24 was found to bind to the central region of MDM2, resulting in the polyubiquitylation of DDX24 both in vitro and in vivo. Unexpectedly, however, the polyubiquitylation of DDX24 did not elicit its proteasomal degradation but rather promoted its association with preribosomal ribonucleoprotein (pre-rRNP) processing complexes that are required for the early steps of pre-rRNA processing. Consistently with these findings, depletion of DDX24 in cells impaired pre-rRNA processing and resulted both in abrogation of MDM2 function and in consequent p53 stabilization. Our results thus suggest an unexpected role of MDM2 in the nonproteolytic ubiquitylation of DDX24, which may contribute to the regulation of pre-rRNA processing.

ACKNOWLEDGMENTS

This work was supported by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. T.Y. was also supported by a research fellowship of the Japan Society for the Promotion of Science for Young Scientists.

We thank Masatoshi Kitagawa for HCT116 cells; Hiroyuki Miyoshi, Hiromitsu Nakauchi, Yohei Kobayashi, and Shin Yonehara for vectors; Naoko Nishimura, Kayoko Tsunematsu, Koji Oyamada, Emiko Koba, and Mizuho Oda for technical assistance; Akane Ohta for help with preparation of the manuscript; and Katsuyoshi Mihara, Shigeki Furuya, and Kohichi Kawahara for both technical assistance and helpful discussion.