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Projects in our lab center on mechanisms of translation control in mammalian cells. Enteroviruses such as poliovirus and coxsackievirus shut off cap-dependent protein synthesis (translation) within 2 hours of infection while allowing cap-independent translation of mRNAs containing IRES elements to continue. Cap-dependent translation is used to produce 96% of all cellular proteins, and the loss of protein homeostasis in the cell is a major cause of cell death.  We have shown that enteroviral 2A protease (2Apro) and 3C protease (3Cpro) are viral proteins required for translation shutoff. 2Apro cleaves the translation initiation factor eIF4G. eIF4G is the central protein involved in initiation of cap-dependent translation since it binds to the 5´ end of capped mRNA and provides a molecular bridge that binds mRNA to 40S ribosomal subunits. Cleavage destroys its ability to function in cap-dependent translation initiation. We have also discovered that enterovirus 2A pro and 3C pro both cleave poly(A)-binding protein (PABP) in infected cells. The cleavage site for both proteases lies in the C-terminal domain of PABP in a region whose function has not been determined. This region somehow regulates ribosome termination, re-initiation and mRNA stability, but these important mechanisms are only beginning to emerge. Thus, the virus attacks proteins which binds to both ends of mRNA, supporting the importance of 5´-3´ interactions that circularize mRNA in translation regulation. Work is continuing to determine the precise function of 5´-3´ interactions and the role of PABP and eIF4G in regulation.

We are also interested in mechanisms that regulate RNA STRESS GRANULE (SG) and Processing Body (PB) formation in cells, which is an extension of translation regulation. Translational silencing by microRNAs is thought to regulate expression of about 50% of human genes, and may proceed by this stress granule pathway.  We showed that poliovirus 3Cpro causes cleaves a protein called G3BP that is critical for nucleating SG formation. We are performing experiments to discern why enteroviruses need to disrupt stress granule formation and how G3BP functions in translation silencing.  We have also shown that PBs are degraded in infected cells and are currently focusing on degradation of Xrn1 and PAN3, both of which regulate RNA decay in cells.

Though we use a virus model system, however much of our work centers on regulation mechanisms in uninfected cells and is now characterizing translational gene regulation in breast cancer.  One project (in collaboration with the Eric Chang lab), investigates functions of Int6 or eIF3e, which we propose dually regulates proteasome function and translation regulation. This gene is important in breast cancer and is frequently mutated into truncated forms.

Recent Publications (PubMed)

Kuyumcu-Martinez NM, Van Eden ME, Younan P, Lloyd RE. (2004) Cleavage of Poly(A)-Binding Protein By Poliovirus 3C Protease Inhibits Host Cell Translation: A Novel Mechanism for Host Translation Shutoff. Mol. Cell Biol. 24: 1779-90.

Kuyumcu-Martinez M, Belliot G, Sosnovtsev SV, Chang KO, Green KY, Lloyd RE. (2004) The calicivirus 3CL proteinase inhibits cellular translation by cleavage of poly(A)-binding protein. J.Virol. 78: 8172-8182

Sherrill K, Van Eden ME, Lloyd RE. (2004) Bcl-2 translation is mediated via internal ribosome entry during cell stress. J. Biol. Chem. 279: 29066-29074.

VanEden ME, Byrd MP, Sherrill K,Lloyd RE. (2004) Translation of cellular inhibitor of apoptosis protein 1 (c-IAP1) mRNA is IRES mediated and regulated during cell stress. RNA 10: 469-481.

Byrd, M.P., M. Zamora and Lloyd RE. (2005) Translation of eukaryotic translation initiation factor 4GI (eIF4GI) proceeds from multiple mRNAs containing a novel cap-dependent IRES that is active during poliovirus infection. J. Biol. Chem. 280: 18610-22.

Lloyd RE. (2006) Translational control by viral proteinases. Virus Res. 119:76-88.

White, J., A.M. Cardenas, W.E. Marissen and R.E. Lloyd.  (2007) Inhibition of cytoplasmic mRNA stress granule formation by a viral proteinase.   Cell Host & Microbe 2: 295-305.

Bonderoff, J.M. and R.E. Lloyd. 2008.  CVB Translation: Lessons from the polioviruses. p123-147.In Current Topics in Microbiology and Immunology: The Group B Coxsackieviruses.  S. Tracy, S. Oberste and K. Drescher eds., Springer-Verlag.

Sherrill, K.W., M.P. Byrd and R.E. Lloyd.  2008. Translation of the transcript encoding cIAP2 is mediated exclusively by a stress-modulated ribosome shunt. Mol. Cell. Biol. 28:2011-2022.

Rivera, C. and R.E. Lloyd.  2008.  Modulation of enteroviral proteinase cleavage of poly(A)-binding protein (PABP) by PABP-associated factors. Virology 375: 59-72.

de Breyne, S.,  J.M. Bonderoff, K.M. Chumakov, R.E. Lloyd, and C.U.T. Hellen. 2008.  Cleavage of eukaryotic initation factor eIF5B by enterovirus 3C proteases. Virology 378: 118-22.

Bonderoff, J.M., J.L. LaRey and R.E. Lloyd. 2008. Cleavage of poly(A)-binding protein by poliovirus 3C proteinase inhibits viral IRES-mediated translation. J. Virol. 82: 9389-99.