Volume 272, Number 33, Issue of August 15, 1997 pp. 20463-20470
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Mitogen-activated Protein Kinase and Cyclin B/Cdc2 Phosphorylate Xenopus Nuclear Factor 7 (xnf7) in Extracts from Mature Oocytes
IMPLICATIONS FOR REGULATION OF xnf7 SUBCELLULAR LOCALIZATION

(Received for publication, April 23, 1997)

Heithem M. El-Hodiri , Shaoli Che ^¶ , Mayra Nelman-Gonzalez ^¶ , Jian Kuang ^¶ and Laurence D. Etkin

From the Departments of  Molecular Genetics and ^¶ Clinical Investigations, University of Texas M.D. Anderson Cancer Center, Houston, Texas 77030

Xenopus nuclear factor 7 (xnf7) is a maternally expressed putative transcription factor that exhibits phosphorylation-dependent changes in subcellular localization during early Xenopus development. Xnf7 is localized to the germinal vesicle (nucleus) of immature oocytes in a hypophosphorylated state. Xnf7 is phosphorylated during oocyte maturation and released to the cytoplasm. The protein is retained in the cytoplasm during early embryonic cleavage stages but returns to nuclei at the mid-blastula transition. Xnf7 is phosphorylated at two sites during oocyte maturation, designated P1, consisting of one threonine at position 103, and P2, consisting of three clustered threonines at positions 209, 212, and 218. Phosphorylation of both sites is important in regulating xnf7 localization. The P1 site can be phosphorylated by cyclin B/Cdc2 in vitro. To further understand the mechanisms regulating subcellular localization of xnf7 during early development, kinases capable of catalyzing phosphorylation of the P2 site were purified from mature oocyte extracts. We found that mitogen-activated protein kinase phosphorylated Thr²¹² and cyclin B/Cdc2 phosphorylated Thr ²⁰⁹ and Thr²¹². No other kinase in mature oocyte extracts phosphorylated the xnf7 P2 site to a significant extent. These results implicate mitogen-activated protein kinase and cyclin B/Cdc2 in regulating xnf7 localization during oocyte maturation. This also suggests that localization of xnf7 may be regulated by multiple kinase activation pathways.

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