title: C-terminal residues of skeletal muscle calsequestrin are essential for calcium binding and for skeletal ryanodine receptor inhibition.
abstract: BACKGROUND
  Skeletal muscle function depends on calcium signaling proteins in the sarcoplasmic reticulum (SR), including the calcium-binding protein calsequestrin (CSQ), the ryanodine receptor (RyR) calcium release channel, and skeletal triadin 95 kDa (trisk95) and junctin, proteins that bind to calsequestrin type 1 (CSQ1) and ryanodine receptor type 1 (RyR1). CSQ1 inhibits RyR1 and communicates store calcium load to RyR1 channels via trisk95 and/or junctin.
  
  
  METHODS
  In this manuscript, we test predictions that CSQ1's acidic C-terminus contains binding sites for trisk95 and junctin, the major calcium binding domain, and that it determines CSQ1's ability to regulate RyR1 activity.
  
  
  RESULTS
  Progressive alanine substitution of C-terminal acidic residues of CSQ1 caused a parallel reduction in the calcium binding capacity but did not significantly alter CSQ1's association with trisk95/junctin or influence its inhibition of RyR1 activity. Deletion of the final seven residues in the C-terminus significantly hampered calcium binding, significantly reduced CSQ's association with trisk95/junctin and decreased its inhibition of RyR1. Deletion of the full C-terminus further reduced calcium binding to CSQ1 altered its association with trisk95 and junctin and abolished its inhibition of RyR1.
  
  
  CONCLUSIONS
  The correlation between the number of residues mutated/deleted and binding of calcium, trisk95, and junctin suggests that binding of each depends on diffuse ionic interactions with several C-terminal residues and that these interactions may be required for CSQ1 to maintain normal muscle function.
authors: John Curtin School of Medical Research, Australian National University, Garran Road, Canberra, ACT 2601 Australia ; Discipline of Biomedical Sciences, Centre for Research in Therapeutic Solutions, Faculty of Education Science, Technology and Maths, University of Canberra, Kirinari Street, Bruce, ACT 2601 Australia.
authors: Nicole A
authors: NA
authors: Beard
authors: John Curtin School of Medical Research, Australian National University, Garran Road, Canberra, ACT 2601 Australia.
authors: Angela F
authors: AF
authors: Dulhunty
keywords: Ca2+ binding protein; Calsequestrin; Ryanodine receptor; Sarcoplasmic reticulum; Skeletal muscle