Differential functional interaction of two Vesl/Homer protein isoforms with ryanodine receptor type 1: a novel mechanism for control of intracellular calcium signaling
Introduction
Control of the intracellular free Ca2+ concentration is crucial for all cells, including neurons and myocytes [7], [8], [9]. Important physiological functions are mediated by subtle changes in the patterns of cytosolic Ca2+ concentration transients. Intracellular calcium channels, such as ryanodine receptors (RyRs), contribute crucially to the generation of these intracellular Ca2+ signals. Here, we show that one group of the binding proteins of ryanodine receptor type 1 (RyR1), the Vesl/Homer family of proteins, regulates channel activity. We analyzed the biophysical properties and cell physiological function of RyR1 in the presence and absence of V-1L and V-1S. V-1L/V-1S and RyR1 are co-expressed in the mammalian CNS and skeletal muscle as shown previously [9], [10], [11], [12]. Vesl/Homer proteins link cellular signaling proteins, such as metabotropic glutamate receptors, Shank, inositol 1,4,5-trisphosphate (IP3) receptors, RyRs, N-type calcium channels, and M-type potassium channels physically [1], [2], [3]. V-1L also regulates cellular differentiation, such as axon pathfinding and growth cone development in neurons [4]. In addition, the direct modulation of G-protein-coupled receptors conveying extra- to intracellular signal transduction by Vesl/Homer proteins has recently been reported [1], [2], [3], [4], [5], [12], [13], [14], [15], [16]. Similarly, different Vesl/Homer isoforms have been identified in both skeletal and cardiac muscle, and a potential function in muscle physiology has been discussed [10], [11]. V-1S is the product of a splicing variant of V-1L that lacks the C-terminal portion of V-1L responsible for a coiled-coil domain-mediated dimerization. V-1S is expressed and regulated in the hippocampus during long-term potentiation (LTP) and therefore potentially involved in memory formation [12], [13], [14], [22].
Section snippets
Experimental procedures
All experiments described in the present study were carried out in accordance with the appropriate NIH and University of North Texas Health Science Center guidelines.
Results
The Vesl/Homer protein binding site PPXXFr had been described by Tu et al. [17]. We found three such proline-rich Vesl/Homer protein binding sites on mammalian RyR1 using the SWISS-PROT protein knowledgebase (http://www.expasy.ch/sprot/; Fig. 1a). RyR1, V-1L, and V-1S [7], [10], [12] proteins that are co-expressed in skeletal muscle and several other cell types also showed a direct protein–protein interaction. RyR1 purified from skeletal muscle SR was incubated with purified GST-labeled V-1L or
Discussion
Taking together the data from the Ca2+ release studies, the electrophysiology and optical imaging experiments, the present study provides evidence for a differential regulation of RyR1 Ca2+ channel function by members of the Vesl/Homer protein family, V-1L and V-1S. The described mechanisms that render RyR1 more active through binding of the scaffold protein V-1L and that can be counteracted by V-1S have important implications for a number of physiological processes. Competition of V-1S and
Acknowledgments
We thank M. Koulen and S. Koulen for generous support and encouragement and Dr. E.E. Quist for critically reading and improving the manuscript. This study was supported by the Ministero dell’Università e della Ricerca Scientifica e Tecnologica (2001–2003 Programma di ricerca di rilevante interesse nazionale on Biopathology of the skeletal muscle fiber) (P.V.), the University of North Texas Health Science Center at Fort Worth Intramural Research Program (P.K.), and a Young Investigator Award
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These authors contributed equally to the work and are listed in alphabetical order.