CALCIUM SIGNALING Inhibited by ERp44: An Inside Job

Higo et al. report a novel inhibition of the inositol trisphosphate receptor (IP₃R), specifically IP₃R1, that acts from the lumenal side of the endoplasmic reticulum (ER). They used an intralumenal domain of the IP₃R1 (1L3V) that diverges among the subtypes to affinity purify interacting proteins from mouse cerebellar microsome preparations. The ER protein ERp44 [identified by matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry] bound to 1L3V under acidic conditions. Further analysis showed coimmunoprecipitation of the proteins from Hela cells treated with cross-linking agents and an interaction between ERp44 and 1L3V under reducing conditions. Using cells transfected to express hemaglutinin (HA)-tagged ERp44 and green fluorescent protein (GPP)-tagged IP₃R1, the interaction, based on coimmunoprecipitation, also increased when internal calcium stores were depleted. Overexpression of ERp44 inhibited calcium signals (measured with the fluorescent calcium indicator Fura-2) in cells expressing IP₃R1 but not in cells lacking IP₃R1 (that is, cells expressing only IP₃R2 and IP₃R3). This inhibitory effect of ERp44 was confirmed by measuring single-channel currents of IP₃R1 in planar bilayers. RNA interference to decrease ERp44 in Hela cells also confirmed an inhibitory role for ERp44 in the cellular calcium response to purinergic receptor activation in IP₃R1-expressing cells. The data suggest that ERp44 is an intralumenal regulator of IP₃R1 and thus of calcium release from the ER in response to changes in lumenal pit, redox state, and calcium concentration.

T. Higo, M. Hattori, T. Nakamura, T. Natsume, T. Michikawa, K. Mikoshiba, Subtype-specific and ER lumenal environment-dependent regulation of inositol 1,4,5-trisphosphate receptor type 1 by ERp44. Cell 120, 85-98 (2005). [PubMed]