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J. Biol. Chem., Vol. 280, Issue 31, 28251-28264, August 5, 2005

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Changes in Ribosomal Binding Activity of eIF3 Correlate with Increased Translation Rates during Activation of T Lymphocytes^*

Suzanne Miyamoto¶, Purvi Patel, and John W. B. Hershey

From the Department of Biochemistry and Molecular Medicine, School of Medicine, University of California-Davis, Davis, California 95616 and the Division of Hematology/Oncology, Cancer Center, University of California-Davis, Sacramento, California 95817

The rate of protein synthesis in quiescent peripheral blood T lymphocytes increases dramatically following mitogenic activation. The stimulation of translation is due to an increase in the rate of initiation caused by the regulation of initiation factor activities. Here, we focus on eIF3, a large multiprotein complex that plays a central role in the formation of the 40 S initiation complex. Using sucrose density gradient centrifugation to analyze ribosome complexes, we find that most eIF3 is not bound to 40 S ribosomal subunits in unactivated T lymphocytes but becomes ribosome-bound following activation. Immunoblot analyses of sucrose gradient fractions for individual eIF3 subunits show that the small eIF3j subunit is unassociated with the eIF3 complex in quiescent T lymphocytes, but upon activation joins the other eIF3 subunits and binds 40 S ribosomal subunits. Because eIF3j has been shown to be required for eIF3 binding to 40 S ribosomes in vitro, the results suggest that mitogenic stimulation of T lymphocytes leads to an activation of eIF3j, thereby enabling eIF3 to bind to the larger ribosome-free eIF3 subunit complex, and then to the 40 S ribosomes. The association of eIF3j with the other eIF3 subunits appears to be inhibited by rapamycin, suggesting a mechanism that lies downstream from the mammalian target of rapamycin kinase. This association requires ionomycin together with a phorbol ester, which also suggests that calcium signaling is involved. We conclude that the complex formation of eIF3 and its association with the ribosomes might contribute to increased translation rates during T lymphocyte activation.

Received for publication, December 15, 2004 , and in revised form, May 24, 2005.

^* This work was supported by National Institutes of Health Grant GM22135 (to J. W. B. H.) and by the Children Miracle Network, UC Davis Health Systems (to S. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ To whom correspondence should be addressed: Division of Hematology/Oncology, UC Davis Cancer Center, 4501 X St., Rm. 3016, Sacramento, CA 95817. Tel.: 916-734-3769; Fax: 916-734-6415; E-mail: smiyamot{at}ucdavis.edu.

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