title: RACK1 inhibits TRPM6 activity via phosphorylation of the fused alpha-kinase domain.
abstract: BACKGROUND
  The maintenance of the body's Mg(2+) balance is of great importance because of its involvement in numerous enzymatic systems and its intervention in neuromuscular excitability, protein synthesis, and nucleic acid stability. Recently, the transient receptor potential melastatin 6 (TRPM6) was identified as the gatekeeper of active Mg(2+) transport and therefore plays a crucial role in the regulation of Mg(2+) homeostasis. Remarkably, TRPM6 combines a Mg(2+) channel with an alpha-kinase domain whose function remains elusive.
  
  
  RESULTS
  Here, we identify the receptor for activated C-kinase 1 (RACK1) as the first regulatory protein of TRPM6 that associates with the alpha-kinase domain. RACK1 and TRPM6 are both present in renal Mg(2+)-transporting distal convoluted tubules. We demonstrate that RACK1 inhibits channel activity in an alpha-kinase activity-dependent manner, whereas small interference (si) RNA-mediated knockdown of RACK1 increases the current. Moreover, threonine(1851) in the alpha-kinase domain was identified as an autophosphorylation site of which the phosphorylation state is essential for the inhibitory effect of RACK1. Importantly, threonine(1851) was crucial for the Mg(2+) sensitivity of TRPM6 autophosphorylation and channel activity. TRPM6 channel activity was less sensitive to Mg(2+) when RACK1 was knocked down by siRNA. Finally, activation of protein kinase C by phorbol 12-myristate 13-acetate-PMA prohibited the inhibitory effect of RACK1 on TRPM6 channel activity.
  
  
  CONCLUSIONS
  We propose a unique mode of TRPM6 regulation in which the Mg(2+) influx is controlled by RACK1 through its interaction with the alpha-kinase and the phosphorylation state of the threonine(1851) residue.
authors.affiliation: Department of Physiology, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, 6500 HB Nijmegen, The Netherlands.
authors.forename: Gang
authors.initials: G
authors.lastname: Cao
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Thébault
authors.forename: Jenny
authors.initials: J
authors.lastname: van der Wijst
authors.forename: AnneMiete
authors.initials: A
authors.lastname: van der Kemp
authors.forename: Edwin
authors.initials: E
authors.lastname: Lasonder
authors.forename: René J M
authors.initials: RJ
authors.lastname: Bindels
authors.forename: Joost G J
authors.initials: JG
authors.lastname: Hoenderop
mesh_terms: D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D002462:Cell Membrane / Q000201:enzymology / Q000378:metabolism; D005786:Gene Expression Regulation; D015723:Gene Library; D006801:Humans; D007668:Kidney / Q000378:metabolism; D051379:Mice; D010766:Phosphorylation; D010770:Phosphotransferases / Q000378:metabolism*; D011485:Protein Binding; D011493:Protein Kinase C / Q000378:metabolism; D017434:Protein Structure, Tertiary; D000075370:Receptors for Activated C Kinase; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism*; D050053:TRPM Cation Channels / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020798:Two-Hybrid System Techniques
chemical_list: D000075370:Receptors for Activated C Kinase; D011956:Receptors, Cell Surface; D050053:TRPM Cation Channels; D010770:Phosphotransferases; D011493:Protein Kinase C