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J. Biol. Chem., Vol. 279, Issue 32, 33890-33898, August 6, 2004

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Interactions of Arabidopsis RS Domain Containing Cyclophilins with SR Proteins and U1 and U11 Small Nuclear Ribonucleoprotein-specific Proteins Suggest Their Involvement in Pre-mRNA Splicing^*

Zdravko J. Lorkovi, Sergiy Lopato, Monika Pexa, Reinhard Lehner, and Andrea Barta

From the Max F. Perutz Laboratories, University Departments at the Vienna Biocenter, Department of Biochemistry, Medical University of Vienna, Dr. Bohrgasse 9/3, A-1030 Vienna, Austria

Ser/Arg (SR)-rich proteins are important splicing factors in both general and alternative splicing. By binding to specific sequences on pre-mRNA and interacting with other splicing factors via their RS domain they mediate different intraspliceosomal contacts, thereby helping in splice site selection and spliceosome assembly. While characterizing new members of this protein family in Arabidopsis, we have identified two proteins, termed CypRS64 and CypRS92, consisting of an N-terminal peptidyl-prolyl cis/trans isomerase domain and a C-terminal domain with many SR/SP dipeptides. Cyclophilins possess a peptidyl-prolyl cis/trans isomerase activity and are implicated in protein folding, assembly, and transport. CypRS64 interacts in vivo and in vitro with a subset of Arabidopsis SR proteins, including SRp30 and SRp34/SR1, two homologs of mammalian SF2/ASF, known to be important for 5' splice site recognition. In addition, both cyclophilins interact with U1–70K and U11–35K, which in turn are binding partners of SRp34/SR1. CypRS64 is a nucleoplasmic protein, but in most cells expressing CypRS64-GFP fusion it was also found in one to six round nuclear bodies. However, co-expression of CypRS64 with its binding partners resulted in re-localization of CypRS64 from the nuclear bodies to nuclear speckles, indicating functional interactions. These findings together with the observation that binding of SRp34/SR1 to CypRS64 is phosphorylation-dependent indicate an involvement of CypRS64 in nuclear pre-mRNA splicing, possibly by regulating phosphorylation/dephosphorylation of SR proteins and other spliceosomal components. Alternatively, binding of CypRS64 to proteins important for 5' splice site recognition suggests its involvement in the dynamics of spliceosome assembly.

Received for publication, January 12, 2004 , and in revised form, May 14, 2004.

^* This work was supported by Austrian Science Foundation Grants SFB1710 and SFB1711 (to A. 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.

The on-line version of this article (available at http://www.jbc.org) contains Fig. S1.

Present address: Dept. of Plant Science, Waite Campus, PMB 1 Glen Osmond, South Australia, 5064, Australia.

To whom correspondence should be addressed. Tel.: 43-1-4277-61642; Fax: 43-1-4277-9616; E-mail: zdravko.lorkovic{at}univie.ac.at or zdravko.lorkovic{at}meduniwien.ac.at.

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