Structure de la sous-unité β du canal calcique : la place du β-interaction domain

Abstract

La structure du canal calcique, voie d’entrée privilégiée des ions calcium dans les cellules excitables, a fait l’objet de nombreuses études de biochimie et de mutagenèse associées à des approches fonctionnelles qui, même en l’absence de données cristallographiques, permettent de percevoir le mode de fonctionnement de ce pore. Au contraire, les données relatives à sa sousunité régulatrice la plus importante, la sousunité β, sont plutôt fragmentaires. Celle-ci joue pourtant un rôle fondamental dans l’adressage, la régulation et les propriétés biophysiques les plus intimes du canal. Ces lacunes sont maintenant partiellement comblées par la publication de plusieurs articles sur la structure tridimensionnelle de la sous-unité β. Cette structure, si elle confirme l’appartenance de la sous-unité ß à la famille des guanylate kinases associées à la membrane (MAGUK), semble néanmoins remettre en cause certaines des données que l’on croyait pourtant fermement établies. Elle laisse aussi entrevoir de nouvelles fonctions dans l’assemblage et la localisation d’un canal fonctionnel.

Voltage-gated calcium channels are key players in a number of fundamental physiological functions including contraction, secretion, transmitter release or gene activation. They allow a flux of calcium into the cell that constitutes a switch-on signal for most of these functions. The structures responsible for the shaping of these fluxes by the membrane voltage belong to the channel itself, but a number of associated proteins are known to more precisely tune this calcium entry and adapt it to the cellular demand. The calcium channel regulatory β subunit is undoubtedly the most important one, being influent on the expression, the kinetics, the voltagedependence of channel opening and closing and on the pharmacology of the channel. Heterologous expression, combined to mutagenesis and electrophysiological and biochemical experiments have revealed the roles of short sequences of the β subunit, including the BID (β-interaction domain), in the physical and functional interactions with the channel pore. The resolved crystal structure of the β subunit now sheds new light on these sequences and their interactions with the rest of the protein. The presence of a type 3 src-homology (SH3) domain and a guanylate kinase (GK) domain confirms that the subunit belongs to the MAGUK protein family. Consistently, the polyproline binding site and the kinase function of the SH3 and the GK domains, respectively, are non functional, and the BID appears to be buried in the structure, preserving the SH3-GK interaction but not directly available for interactions with the channel pore subunit. Anchoring of the β subunit to the channel occurs via a hydrophobic grove in the GK domain, leaving a large surface of the subunit open to other protein-protein interactions. To what extent the intramolecular SH3-GK interaction is necessary for the stabilisation of this grove in a functional unit remains to be understood. The β subunit may thus play a key role in scaffolding multiple proteins around the channel and organizing diverse calcium- dependent signalling pathways directly linked to voltagegated calcium entry. These findings will undoubtedly vitalize the search for new β-specific partners and functions.