Calreticulin Modulates Capacitative Ca2+ Influx by Controlling the Extent of Inositol 1,4,5-Trisphosphate-induced Ca2+ Store Depletion

Wen Xu,Frank J Longo,Mary R Wintermantel,Xueying Jiang,Robert A Clark,Sylvain Delisle

doi:10.1074/jbc.m002041200

Wen Xu, Frank J Longo + Show 4 more

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https://doi.org/10.1074/jbc.m002041200

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Abstract

Calreticulin (CRT) is a highly conserved Ca(2+)-binding protein that resides in the lumen of the endoplasmic reticulum (ER). We overexpressed CRT in Xenopus oocytes to determine how it could modulate inositol 1,4,5-trisphosphate (InsP(3))-induced Ca(2+) influx. Under conditions where it did not affect the spatially complex elevations in free cytosolic Ca(2+) concentration ([Ca(2+)](i)) due to InsP(3)-induced Ca(2+) release, overexpressed CRT decreased by 46% the Ca(2+)-gated Cl(-) current due to Ca(2+) influx. Deletion mutants revealed that CRT requires its high capacity Ca(2+)-binding domain to reduce the elevations of [Ca(2+)](i) due to Ca(2+) influx. This functional domain was also required for CRT to attenuate the InsP(3)-induced decline in the free Ca(2+) concentration within the ER lumen ([Ca(2+)](ER)), as monitored with a "chameleon" indicator. Our data suggest that by buffering [Ca(2+)](ER) near resting levels, CRT may prevent InsP(3) from depleting the intracellular stores sufficiently to activate Ca(2+) influx.

Highlights

Calreticulin (CRT) is a highly conserved Ca2؉-binding protein that resides in the lumen of the endoplasmic reticulum (ER)
Our data suggest that by buffering [Ca2؉]ER near resting levels, CRT may prevent InsP3 from depleting the intracellular stores sufficiently to activate Ca2؉ influx. Since it was first isolated a quarter of a century ago [1], the protein calreticulin (CRT)1 has been identified in a great variety of cells, implying an essential biological activity [2]
Stimulation of Ca2ϩ influx appears closely linked to the depletion of the intracellular Ca2ϩ stores, a relationship known as capacitative Ca2ϩ entry

Summary

Introduction

Calreticulin (CRT) is a highly conserved Ca2؉-binding protein that resides in the lumen of the endoplasmic reticulum (ER). Our data suggested that this was not the case with saturating concentrations of InsP3; in the cells where CRT had decreased the Ca2ϩ influx-related ClϪ current, there was no change in the initial ClϪ currents due to IICR.

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