MICHAEL WORMINGTON

Education

• B.A., Biology, University of Kansas, 1975
• Ph.D., Biochemistry, University of Kansas, 1979
• Postdoctoral: Carnegie Inst. Dept. of Embryology, 1979-1982

Contact Information

Research Interests

Developmental Regulation of mRNA Silencing, Surveillance and Turnover

Deadenylation-Dependent Translational Silencing of Maternal mRNAs

Oocytes synthesize and accumulate a large pool of maternal mRNAs. Some of these are actively utilized for protein synthesis throughout oogenesis and subsequently silenced during meiotic maturation or at fertilization. Conversely, other mRNAs are repressed in immature oocytes and either transiently or stably activated at later developmental stages. This translational control is the principal mechanism to regulate protein synthesis before the onset of zygotic transcription and underlies many crucial developmental events (Figure 1). The developmentally regulated removal of 3' poly(A) tails is a critical and evolutionarily conserved determinant of maternal mRNA silencing. Our research addresses the molecular basis for the deadenylation-dependent translational inactivation of maternal mRNAs during the meiotic maturation or fertilization of Xenopus oocytes (Figure 2). Deadenylation in mature oocytes is catalyzed by the poly(A)-specific ribonuclease, PARN. PARN activity is stimulated by the presence of the 5' m7GpppG cap on mRNAs and cap recognition is an intrinsic property of this nuclease. Thus, PARN deadenylation activity is intimately linked with translation efficiency as the 5' cap and 3' poly(A) tail are major determinants of both processes. These reactions are necessarily antagonistic as the relevant cap- and poly(A)-binding proteins, eIF4E and PABP, synergistically promote efficient translation initiation and inhibit PARN (Figure 3). Conversely, PARN potentially "invades" this closed loop through its catalytic, cap- and poly(A)-binding activities thereby silencing translation. We are utilizing complementary molecular biological and biochemical in vivo and in vitro approaches to delineate functional domains within PARN and to examine functional and physical interactions between PARN and cap-dependent translation initiation factors. Deadenylation of Xenopus maternal mRNAs is not restricted to mature oocytes. Certain mRNAs contain specific elements within their 3' untranslated regions which promote their deadenylation after fertilization, thereby restricting their translation to mature oocytes (Figure 2). We are determining if PARN participates in post-fertilization deadenylation or if the Xenopus orthologues of the yeast deadenylases, CCR4 and POP2, catalyze this reaction.

Developmental Regulation of mRNA Decapping Activities

In contrast to yeast and metazoan somatic cells in which poly(A) removal triggers subsequent decapping and exonucleolytic decay events (Figure 4), deadenylated maternal mRNAs persist for several hours until zygotic transcription begins at the midblastula transition (MBT). The absence of a deadenylation-dependent decapping activity in oocytes could contribute to this stabilization. We have cloned Xenopus orthologues of two distinct decapping activities, Dcp1 and DcpS, and are characterizing their regulation.

CBTF: A Multifunctional Transcriptional and Translational Regulatory Protein

The onset of zygotic transcription is controlled by the temporal and spatial regulation of maternal mRNAs and proteins synthesized during oogenesis. Transcriptional activation of the hematopoietic regulatory factor, GATA-2, by the maternally inherited CCAAT box transcription factor, CBTF, illustrates how maternal and zygotic programs of gene expression are integrated. CBTF is initially present in the nucleus of immature oocytes. CBTF is released into the cytoplasm during oocyte maturation where it binds to translationally repressed maternal mRNAs and associates with the double-stranded RNA-activated protein kinase, PKR, a negative regulator of global translation. RNA-binding also anchors CBTF within the cytoplasm and prevents its nuclear translocation and ability to activate GATA-2 transcription during early development. The nuclear localization of CBTF at the MBT is coupled with the degradation of deadenylated maternal mRNAs at this stage. We are addressing the molecular basis of this unique mechanism in which nuclear import of a transcriptional activator that is also an RNA binding protein, couples zygotic gene expression with maternal mRNA degradation.

Representative Publications

1. Wormington, M. (2003) Zero tolerance for nonsense: Nonsense-mediated mRNA decay uses multiple degradation pathways. Molecular Cell 12:536 - 538.
   
   
2. Copeland, P.R. and Wormington, M. (2001) The mechanism and regulation of deadenylation: Identification and characterization of Xenopus PARN. RNA 7:675-686. Abstract
   
   

3. Wilusz, C.J., Wormington, M. and Peltz, S.W. (2001) The cap-to-tail guide to mRNA turnover. Nature Reviews: Molecular Cell Biology 2:237-246. Abstract

4. Brzostowski, J.A., Robinson, C.A., Orford, R., Elgar, S., Scarlett, G., Peterkin, T., Malartre, M., Kneale, G., Wormington, M. and Guille, M. (2000) RNA-dependent cytoplasmic anchoring of a transcription factor subunit during early Xenopus development. EMBO J. 19:3683-3693. Abstract

5. Dehlin, E., Wormington, M., Körner, C.G. and Wahle, E. (2000) Cap-dependent deadenylation of mRNA. EMBO J. 19:1079-1086. Abstract

6. Zhang, S., Williams, C.J., Wormington, M., Stevens, A., and Peltz, S.W. (1999) Monitoring mRNA decapping activity. Methods: A Companion to Methods in Enzymology 17:46-51. Abstract

7. Körner, C.G., Wormington, M., Muckenthaler, M., Schneider, S., Dehlin, E., and Wahle, E. (1998) The deadenylating nuclease (DAN) is involved in poly(A) removal during the meiotic maturation of Xenopus oocytes. EMBO J. 17:5427-5437. Abstract

8. Meric, F., Searfoss, A.M., Wormington, M. and Wolffe, A.P. (1996) Masking and unmasking maternal mRNA: the role of polyadenylation, transcription, splicing and nuclear history. J. Biol. Chem. 271:30804-30810. Abstract

9. Wormington, M., Searfoss, A.M. and Hurney, C.A. (1996) Overexpression of poly(A) binding protein prevents maturation-specific deadenylation and translational inactivation in Xenopus oocytes. EMBO J. 15: 900-909. Abstract