title: Domain cooperativity in the β1a subunit is essential for dihydropyridine receptor voltage sensing in skeletal muscle.
abstract: The dihydropyridine receptor (DHPR) β1a subunit is crucial for enhancement of DHPR triad expression, assembly of DHPRs in tetrads, and elicitation of DHPRα1S charge movement--the three prerequisites of skeletal muscle excitation-contraction coupling. Despite the ability to fully target α1S into triadic junctions and tetradic arrays, the neuronal isoform β3 was unable to restore considerable charge movement (measure of α1S voltage sensing) upon expression in β1-null zebrafish relaxed myotubes, unlike the other three vertebrate β-isoforms (β1a, β2a, and β4). Thus, we used β3 for chimerization with β1a to investigate whether any of the five distinct molecular regions of β1a is dominantly involved in inducing the voltage-sensing function of α1S. Surprisingly, systematic domain swapping between β1a and β3 revealed a pivotal role of the src homology 3 (SH3) domain and C terminus of β1a in charge movement restoration. More interestingly, β1a SH3 domain and C terminus, when simultaneously engineered into β3 sequence background, were able to fully restore charge movement together with proper intracellular Ca(2+) release, suggesting cooperativity of these two domains in induction of the α1S voltage-sensing function in skeletal muscle excitation-contraction coupling. Furthermore, substitution of a proline by alanine in the putative SH3-binding polyproline motif in the proximal C terminus of β1a (also of β2a and β4) fully obstructed α1S charge movement. Consequently, we postulate a model according to which β subunits, probably via the SH3-C-terminal polyproline interaction, adapt a discrete conformation required to modify the α1S conformation apt for voltage sensing in skeletal muscle.
authors.affiliation: Department of Medical Genetics, Molecular, and Clinical Pharmacology, Innsbruck Medical University, A-6020 Innsbruck, Austria.
authors.forename: Anamika
authors.initials: A
authors.lastname: Dayal
authors.forename: Vinayakumar
authors.initials: V
authors.lastname: Bhat
authors.forename: Clara
authors.initials: C
authors.lastname: Franzini-Armstrong
authors.forename: Manfred
authors.initials: M
authors.lastname: Grabner
mesh_terms: D020816:Amino Acid Motifs; D000818:Animals; D020746:Calcium Channels, L-Type / Q000737:chemistry* / Q000378:metabolism*; D056966:Excitation Contraction Coupling; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D008954:Models, Biological; D018485:Muscle Fibers, Skeletal / Q000378:metabolism; D018482:Muscle, Skeletal / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D020033:Protein Isoforms / Q000378:metabolism; D021122:Protein Subunits / Q000737:chemistry* / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D013329:Structure-Activity Relationship; D015027:Zebrafish / Q000378:metabolism*; D029961:Zebrafish Proteins / Q000737:chemistry* / Q000378:metabolism*; D018909:src Homology Domains
chemical_list: C509902:CACNB1 protein, zebrafish; D020746:Calcium Channels, L-Type; D020033:Protein Isoforms; D021122:Protein Subunits; D011994:Recombinant Proteins; D029961:Zebrafish Proteins; D049452:Green Fluorescent Proteins