Journal of Cellular Biochemistry 90:1186–1196 (2003) Inositol 1,4,5-Trisphosphate Receptor (Type 1) Phosphorylation and Modulation by Cdc2 Krishnamurthy Malathi, 1 Shinya Kohyama, 1 Michael Ho, 1,3 Damien Soghoian, 1 Xiaogui Li, 1 Michael Silane, 4 Alejandro Berenstein, 5 and Thottala Jayaraman 1,2,3 * 1 Vascular Biology Laboratory, Department of Medicine, St. Luke’s Roosevelt Hospital Center, New York, New York 2 Division of Cardiology, Department of Medicine, New York, New York 3 College of Physicians & Surgeons of Columbia University, New York, New York 4 Division of Vascular Surgery, Beth Israel Medical Center, New York, New York 5 Division of Endovascular Surgery, Beth Israel Medical Center, New York, New York Abstract Calcium (Ca 2þ ) release from the endoplasmic reticulum (ER) controls numerous cellular functions including proliferation, and is regulated in part by inositol 1,4,5-trisphosphate receptors (IP3Rs). IP3Rs are ubiquitously expressed intracellular Ca 2þ -release channels found in many cell types. Although IP3R-mediated Ca 2þ release has been implicated in cellular proliferation, the biochemical pathways that modulate intracellular Ca 2þ release during cell cycle progression are not known. Sequence analysis of IP3R1 reveals the presence of two putative phosphorylation sites for cyclin-dependent kinases (cdks). In the present study, we show that cdc2/CyB, a critical regulator of eukaryotic cell cycle progression, phosphorylates IP3R1 in vitro and in vivo at both Ser 421 and Thr 799 and that this phosphorylation increases IP3 binding. Taken together, these results indicate that IP3R1 may be a specific target for cdc2/CyB during cell cycle pro- gression. J. Cell. Biochem. 90: 1186 – 1196, 2003. ß 2003 Wiley-Liss, Inc. Key words: proliferation; IP3R1; Cdc2; phosphorylation; calcium IP3R-mediated Ca 2þ signaling is involved in modulating cell growth and death pathways [Jayaraman and Marks, 1997; Marks, 1997], and IP3Rs are ubiquitously expressed intracel- lular Ca 2þ -release channels in many cell types [Ehrlich et al., 1994; Marks, 1997]. In mamma- lian tissues, at least three forms of IP3R have been identified. The channel exists as homo- tetrameric and heterotetrameric structures [Joseph et al., 1995; Nucifora et al., 1996] with three functional domains: a transmembrane domain containing the Ca 2þ -channel pore close to the carboxy-terminus, the amino-terminal IP3-binding domain, and a large cytosolic do- main that connects the Ca 2þ channel with the IP3-binding region [Mignery and Sudhof, 1990; Joseph, 1996]. Most of the IP3Rs, excluding a short transmembrane Ca 2þ -channel region, are exposed to the cytoplasm and are targets for several accessory proteins as well as kinases. For instance, IP3R1 functions are modulated by several accessory proteins including the FK-506 binding protein, FKBP12, a member of the immunophilin family of cis – trans pepti- dyl-prolyl isomerases [Cameron et al., 1995b; Poirier et al., 2001], calcineurin [Cameron et al., 1995a], homer protein that binds to a proline-rich motif [Tu et al., 1998], the nonre- ceptor protein tyrosine kinase Fyn [Jayaraman et al., 1996], and inositol 1,4,5-trisphosphate receptor-associated cGMP substrate (IRAG) [Schlossmann et al., 2000]. A homer ligand- like motif is conserved in IP3R1 at amino acids 48–55; the binding regions for Fyn and PKG have not yet been determined. ß 2003 Wiley-Liss, Inc. Grant sponsor: The American Heart Association, the Herbert Irving Comprehensive Center; Grant sponsor: Avon Scholar Pilot Awards in Breast Cancer; Grant sponsor: The Vascular Biology Fund (to TJ); Grant sponsor: The Japanese Defense College (to SK); Grant sponsor: The Medical Students Summer Research Fellowship Program (to MH). *Correspondence to: Thottala Jayaraman, PhD, S&R 1009, Vascular Biology Laboratory, St. Luke’s Roosevelt Hospital, 1111 Amsterdam Avenue, New York, NY 10025. E-mail: tj56@columbia.edu Received 26 March 2003; Accepted 22 August 2003 DOI 10.1002/jcb.10720