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J. Biol. Chem., Vol. 281, Issue 15, 10399-10409, April 14, 2006

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Domain Analysis of a Groundnut Calcium-dependent Protein Kinase

NUCLEAR LOCALIZATION SEQUENCE IN THE JUNCTION DOMAIN IS COUPLED WITH NONCONSENSUS CALCIUM BINDING DOMAINS^*

Ayan Raichaudhuri¹, Rajasri Bhattacharyya², Shubho Chaudhuri³, Pinak Chakrabarti, and Maitrayee DasGupta⁴

From the Department of Biochemistry, Calcutta University, 35, Ballygunge Circular Road, Calcutta 700019 and the Department of Biochemistry and Bioinformatics Centre, Bose Institute, P1/12 CIT Scheme VIIM, Calcutta 700054, India

The signature of calcium-dependent protein kinases (CDPKs) is a C-terminal calmodulin-like domain (CaMLD) with four consensus calcium-binding sites. A junction domain (JD) joins the kinase with CaMLD and interacts with them through its autoinhibitory and CaMLD binding subdomains, respectively. We noted several CDPKs additionally have a bipartite nuclear localization signal (NLS) sequence as a subdomain in their JD, and this feature is obligatorily coupled with the absence of consensus calcium-binding sites in their respective CaMLDs. These predicted features are substantiated by undertaking investigations on a CDPK (gi:67479988) isolated from cultured groundnut (Arachis hypogea) cells. This kinase can bind 3.1 mol of Ca²⁺ under saturating conditions with a considerably high K_d of 392 µM as compared with its canonical counterparts. CD spectroscopic analysis, however, indicates the intramolecular structural changes accompanied with calcium binding to be similar to canonical CDPKs. Attesting to the presence of NLS in the JD, the endogenous kinase is localized in the nucleus of osmotically stressed Arachis cells, and in vitro binding assays indicate the NLS in the JD to interact with nuclear transport factors of the importin family. Homology modeling also indicates the feasibility of interaction of importins with the NLS present in the JD of such CDPKs in their activated form. The possible significance of obligatory coupling between the presence of NLS in the junction domain and atypical calcium binding properties of these CDPKs is discussed in the light of the known mechanisms of activation of these kinases.

Received for publication, October 7, 2005 , and in revised form, January 12, 2006.

^* This work was supported in part by the Council of Scientific and Industrial Research, Government of India, Grant 38(1041)/02/EMR-II, 2002-2003. 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.

¹ Recipient of Fellowship 2-48/2001(i)-EU-II from the University Grants Commission, Government of India.

² Supported by the Department of Biotechnology, Government of India.

³ Present address: School of Molecular Biosciences, Washington State University, Pullman, WA 99164-4660.

⁴ To whom correspondence should be addressed. Tel.: 91-33-24753680; Fax: 91-33-24764419; E-mail: maitrayee_d{at}hotmail.com.

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