Skip to main content
Article thumbnail
Location of Repository

Roles of Tyr122-hydrophobic Cluster and K+ Binding in Ca2+-releasing Process of ADP-insensitive Phosphoenzyme of Sarcoplasmic Reticulum Ca2+-ATPase*S⃞

By Kazuo Yamasaki, Guoli Wang, Takashi Daiho, Stefania Danko and Hiroshi Suzuki


Tyr122-hydrophobic cluster (Y122-HC) is an interaction network formed by the top part of the second transmembrane helix and the cytoplasmic actuator and phosphorylation domains of sarcoplasmic reticulum Ca2+-ATPase. We have previously found that Y122-HC plays critical roles in the processing of ADP-insensitive phosphoenzyme (E2P) after its formation by the isomerization from ADP-sensitive phosphoenzyme (E1PCa2) (Wang, G., Yamasaki, K., Daiho, T., and Suzuki, H. (2005) J. Biol. Chem. 280, 26508–26516). Here, we further explored kinetic properties of the alanine-substitution mutants of Y122-HC to examine roles of Y122-HC for Ca2+ release process in E2P. In the steady state, the amount of E2P decreased so that of E1PCa2 increased with increasing lumenal Ca2+ concentration in the mutants with K0.5 110–320 μm at pH 7.3. These lumenal Ca2+ affinities in E2P agreed with those estimated from the forward and lumenal Ca2+-induced reverse kinetics of the E1PCa2-E2P isomerization. K0.5 of the wild type in the kinetics was estimated to be 1.5 mm. Thus, E2P of the mutants possesses significantly higher affinities for lumenal Ca2+ than that of the wild type. The kinetics further indicated that the rates of lumenal Ca2+ access and binding to the transport sites of E2P were substantially slowed by the mutations. Therefore, the proper formation of Y122-HC and resulting compactly organized structure are critical for both decreasing Ca2+ affinity and opening the lumenal gate, thus for Ca2+ release from E2PCa2. Interestingly, when K+ was omitted from the medium of the wild type, the properties of the wild type became similar to those of Y122-HC mutants. K+ binding likely functions via producing the compactly organized structure, in this sense, similarly to Y122-HC

Topics: Membrane Transport, Structure, Function, and Biogenesis
Publisher: American Society for Biochemistry and Molecular Biology
OAI identifier:
Provided by: PubMed Central
Download PDF:
Sorry, we are unable to provide the full text but you may find it at the following location(s):
  • http://www.pubmedcentral.nih.g... (external link)
  • Suggested articles

    To submit an update or takedown request for this paper, please submit an Update/Correction/Removal Request.