title: Block of T-type calcium channels by protoxins I and II.
abstract: BACKGROUND
  Low-voltage-activated (T-type) calcium channels play a crucial role in a number of physiological processes, including neuronal and cardiac pacemaker activity and nociception. Therefore, finding specific modulators and/or blockers of T-type channels has become an important field of drug discovery. One characteristic of T-type calcium channels is that they share several structural similarities with voltage-gated sodium channels (VGSCs). We therefore hypothesized that binding sites for certain sodium channel blocking peptide toxins may be present in T-type calcium channels.
  
  
  RESULTS
  The sodium channel blocker ProTx I tonically blocked native and transiently expressed T-type channels in the sub- to low micro molar range with at least a ten-fold selectivity for the T-type calcium channel hCav3.1 over hCav3.3, and more than one hundred fold selectivity over hCav3.2. Using chimeras of hCav3.1 and hCav3.3, we determined that the domain IV region of hCav3.1 is a major determinant of toxin affinity, with a minor contribution from domain II. Further analysis revealed several residues in a highly conserved region between T-type and sodium channels that may correspond to toxin binding sites. Mutagenesis of several of these residues on an individual basis, however, did not alter the blocking effects of the toxin. ProTx II on the other hand preferentially blocked hCav3.2 and significantly shifted the steady state inactivation of this channel.
  
  
  CONCLUSIONS
  ProTx I blocks hCav3.1 both selectively and with high affinity. Domain IV appears to play a major role in this selectivity with some contribution from domain II. Given the structural similarities between sodium and T-type calcium channels and the apparent conservation in toxin binding sites, these data could provide insights into the development and synthesis of novel T-type channel antagonists.
authors.forename: Chris
authors.initials: C
authors.lastname: Bladen
authors.forename: Jawed
authors.initials: J
authors.lastname: Hamid
authors.forename: Ivana A
authors.initials: IA
authors.lastname: Souza
authors.affiliation: Department of Physiology & Pharmacology, Hotchkiss Brain Institute, University of Calgary, 3330 Hospital Drive, Calgary, NW, Canada. zamponi@ucalgary.ca.
authors.forename: Gerald W
authors.initials: GW
authors.lastname: Zamponi
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D020747:Calcium Channels, T-Type / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects; D051379:Mice; D008969:Molecular Sequence Data; D062556:NAV1.7 Voltage-Gated Sodium Channel / Q000737:chemistry / Q000378:metabolism; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D010455:Peptides / Q000494:pharmacology*; D017434:Protein Structure, Tertiary; D013111:Spider Venoms / Q000494:pharmacology*; D013329:Structure-Activity Relationship
chemical_list: D002121:Calcium Channel Blockers; D020747:Calcium Channels, T-Type; D062556:NAV1.7 Voltage-Gated Sodium Channel; D010455:Peptides; C470625:ProTx-I peptide; C486866:SCN9A protein, human; D013111:Spider Venoms; C581635:protoxin II, Thrixopelma pruriens