A multifunctional protein that is found primarily within membrane-bound organelles. In the endoplasmic reticulum it binds to specific N-linked oligosaccharides found on newly-synthesized proteins and functions as a MOLECULAR CHAPERONE that may play a role in protein folding or retention and degradation of misfolded proteins. In addition calreticulin is a major storage form for calcium and functions as a calcium-signaling molecule that can regulate intracellular calcium homeostasis.