PMID: 23109337

Authors:

Liu Y, Zheng X, Mueller GA, Sobhany M, Derose EF, Zhang Y, London RE, Birnbaumer L

Title:

Crystal Structure of Calmodulin Binding Domain of Orai1 in Complex with Ca2+/Calmodulin Displays a Unique Binding Mode.

Journal:

J Biol Chem. 2012 Oct 29.

Abstract:

Orai1 is a plasma membrane protein which in its tetrameric form is responsible for calcium influx from the extracellular environment into cytosol in response to interaction with the Ca2+-depletion sensor STIM1. This is followed by a fast Ca2+/calmodulin (CaM)-dependent inhibition, resulting from CaM binding to an Orai1 region called calmodulin binding domain (CMBD). The interaction between Orai1 and CaM at the atomic level remains unknown. Here, we report the crystal structure of a CaM/Orai1-CMBD complex showing one CMBD bound to the C-terminal lobe of CaM, differing from other CaM/target protein complexes, in which both N- and C-terminal lobes of CaM (CaM-N and CaM-C) are involved in target binding. Orai1-CMBD binds CaM-C mainly through hydrophobic interactions, involving primarily residue W76 of Orai1-CMBD which interacts with the hydrophobic pocket of CaM-C. However, NMR data, isothermal titration calorimetry data, and pull-down assays indicated that CaM-N and CaM-C both can bind Orai1-CMBD, with CaM-N having ~4 times weaker affinity than CaM-C. Pull-down assays of a Orai1-CMBD(W76E) mutant, gel filtration chromatography data and NOE signals indicated that CaM-N and CaM-C can each bind one Orai1-CMBD. Thus our studies support an unusual, extended 1:2 binding mode of CaM to Orai1-CMBDs, and quantify the affinity of Orai1 for CaM. We propose a two-step mechanism for CaM-dependent Orai1 inactivation initiated by binding of the C-lobe of CaM to the CMBD of one Orai1 followed by the binding of the N-lobe of CaM to the CMBD of a neighboring Orai1.