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J. Biol. Chem., Vol. 280, Issue 26, 24308-24314, July 1, 2005

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A Novel Class of Dual-family Immunophilins^*

Brian Adams, Alla Musiyenko, Rajinder Kumar, and Sailen Barik

From the Department of Biochemistry and Molecular Biology, University of South Alabama, College of Medicine, Mobile, Alabama 36688-0002

Immunophilins are protein chaperones with peptidylprolyl isomerase activity that belong to one of two large families, the cyclosporin-binding cyclophilins (CyPs) and the FK506-binding proteins (FKBPs). Each family displays characteristic and conserved sequence features that differ between the two families. We report a novel group of dual-family immunophilins that contain both CyP and FKBP domains for which we propose the name FCBP (FK506- and cyclosporin-binding protein). The FCBP of Toxoplasma gondii, a protozoan parasite, contained N-terminal FKBP and C-terminal CyP domains joined by tetratricopeptide repeats. Structure-function analysis revealed that both domains were functional and exhibited family-specific drug sensitivity. The individual domains of FCBP inhibited calcineurin (protein phosphatase 2B) in the presence of the appropriate drugs. In binding studies, FCBP recruited calcineurin in the presence of FK506 and a putative target of rapamycin homolog in the presence of rapamycin. Two additional FCBP sequences in Flavobacterium and one in Treponema (spirochete) were also identified in which the CyP and FKBP domains were in the reverse order. T. gondii growth was inhibited by cyclosporin and FK506 in a moderately synergistic manner. The knockdown of FCBP by RNA interference revealed its essentiality for T. gondii growth. Clearly, the FCBPs are novel chaperones and potential targets of multiple immunosuppressant drugs.

Received for publication, January 26, 2005 , and in revised form, April 19, 2005.

^* This work was supported by NIAID, National Institutes of Health Grant AI045803 (to S. B.). 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. Tel.: 251-460-6860; Fax: 251-460-6127; E-mail: sbarik{at}jaguar1.usouthal.edu.

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