ORGANELLE MARKERS - Endoplasmic Reticulum

Glucose-related protein

Glucose-regulated protein 94, also known as Grp94 or gp96, is an abundant resident endoplasmic reticulum (ER) lumenal stress protein which together with cytosolic Hsp90 belongs to the Hsp90 family of molecular chaperones. Grp94 and other resident soluble proteins of the ER such as members of the Ca(2+) binding protein subfamily (CaBP), CaBPI and CaBP2 as well as calreticulin, possesses the COOH-terminal tetrapeptide Lys-Asp-Glu-Leu (KDEL) which is a sorting signal that is thought to lead to the retention of these proteins in the pre-Golgi compartments. Grp94 expression is upregulated by stress conditions such as glucose starvation and heat shock, which promote protein misfolding or unfolding. In addition to a homeostatic role in protein folding and assembly, Grp94 can function in the intracellular trafficking of peptides from the extracellular space to the MHC class I antigen processing pathway of antigen presentation cells. Grp94
and Hsp90 share high sequence identity and presumably identical adenosine nucleotide-dependent modes of regulation. But earlier data suggests that Hsp90 and Grp94 may differ in their nucleotide binding properties. The Nterminal domain of eukaryotic Hsp90 proteins contains a conserved adenosine nucleotide binding pocket which also serves as the binding site for the Hsp90 inhibitors geldanamycin and radicicol. However, the molecular basis for adenosine nucleotide-dependent regulation of Grp94 remains to be established. Recent data has identified a ligand dependent regulation of Grp94 function and suggest a model whereby Grp94 function is regulated through a ligand-dependent conversion of Grp94 from an inactive to an active conformation.

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