Akira Kaji, Ph.D.

Professor of Microbiology

Office Address:
University of Pennsylvania School of Medicine
141 Anatomy-Chemistry
3610 Hamilton Walk
Philadelphia, PA 19104

TEL 215-898-8828
CELL 215-370-9799
FAX 215-829-9053
kaji@mail.med.upenn.edu

RESEARCH SUMMARY

Protein synthesis consists of four steps; initiation, elongation, termination and recycling of ribosomes for new round of translation. The recycling step is catalyzed by two soluble factors, ribosome recycling factor (RRF) and elongation factor G (EF-G). Our lab discovered this recycling step, RRF (an essential protein for maintenance of bacterial life), and the gene for RRF (frr) (review 1). Our recent work elucidated the structural aspects of the mechanism of the action of RRF 2; 3; 4 In connection with RRF, we study the action of paromomycin, an inhibitor of the RRF reaction 5.

The following is quick review of our past work on protein synthesis. Upon in vivo inactivation of RRF, the ribosome starts the unscheduled translation downstream from the termination codon. Those ribosomes eventually come to the end of mRNA and released but are inactivated for normal protein synthesis, but not for those with leaderless mRNA Crystal structure by X-ray crystallography as well as solution structure by NMR (9) has revealed that RRF is a near perfect mimic of tRNA. Indeed, we found that RRF mimics tRNA in that it moves in the inter-subunits space like tRNA does 2.The mode of RRF binding to the ribosome is different from that of tRNA however. 3.  Fast kinetic analysis of interaction between RRF and the ribosome has been performed. In eukaryotes, RRF is localized in organelles such as chloroplasts and mitochondria. It is therefore possible to design drugs against bacterial RRF .We recently solved this long standing problem in biology. We found that eEF2 (eukaryotic homolog of EF-G) and ATP but not GTP is involved in the ribosome recycling of eukaryotic cytoplasmic protein synthesis. Our classical work is that, we contributed toward the deciphering the genetic code by discovering that specific tRNA binds to the complex of mRNA and ribosomes in 1963.

RECENT PUBLICATIONS (also see Pubmed button below)

1.  Hirokawa, G., Demeshkina, N., Iwakura, N., Kaji, H. & Kaji, A. (2006). The  Ribosome Recycling Step: Consensus or Controversy? Trends Biochem Sci 31, 143-9.

2.  Barat, C., Datta, P. P., Raj, V. S., Sharma, M. R., Kaji, H., Kaji, A. & Agrawal, R. K. (2007). Progression of the ribosome recycling factor through the ribosome dissociates the two ribosomal subunits. Mol Cell 27, 250-261.

3.  Borovinskaya, M. A., Pai, R. D., Zhang, W., Schuwirth, B. S., Holton, J. M., Hirokawa, G., Kaji, H., Kaji, A. & Cate, J. H. (2007). Structural basis for aminoglycoside inhibition of bacterial ribosome recycling. Nat Struct Mol Biol 14, 727-32.

4.  Pai, R. D., Zhang,W., Schuwirth, S.B., Hirokawa, G., Kaji, H., Kaji, A., Cate, J.H.D. (2008). Structural insights into Ribosomal Recycling factor interactions with 70S ribosome. J. Mol. Biol.

5.  Hirokawa, G., Kaji, H. & Kaji, A. (2007). Inhibition of Anti-Association Activity of Translation Initiation Factor 3 by Paromomycin. Antimicrob Agents Chemother 51, 175-80.