Gp96/GRP94 is a putative high density lipoprotein-binding protein in liver

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Three structurally related high-density lipoprotein (HDL)-binding proteins (90, 110, 180 kDa) were previously identified in liver.
• These proteins are now identified as gp96/GRP94, a protein with known endoplasmic reticulum functions and unknown plasma membrane roles.

Purpose of the Study:

• To identify the previously characterized HDL-binding proteins in the liver.
• To investigate the subcellular localization and function of gp96/GRP94, particularly its role in HDL binding.

Main Methods:

• Ultrastructural studies to verify plasma membrane localization of gp96/GRP94.
• Transient and stable transfection of COS-1 cells and other cell types to study protein expression and localization.
• Analysis of HDL-binding activity in transfected cells.

Main Results:

• Gp96/GRP94 was confirmed as the 90, 110, and 180 kDa HDL-binding proteins.
• The homodimeric form of gp96/GRP94 was localized to endosomal/lysosomal vesicles and apical hepatocyte membranes (bile canaliculi).
• Monomeric gp96/GRP94 was found at the basolateral membrane and in coated pits, suggesting a role in receptor-mediated endocytosis. Transient expression in COS-1 cells showed ER localization, while KDEL-deleted constructs showed low-level plasma membrane expression.
• Stable transfection proved difficult, but cells expressing gp96/GRP94 showed increased HDL-binding activity.

Conclusions:

• Gp96/GRP94 exhibits dual subcellular localization, functioning as both an ER chaperonin and a plasma membrane protein involved in HDL binding.
• The protein's presence in coated pits suggests a role in endocytosis.
• Further research is needed to fully elucidate the functions of gp96/GRP94 at the plasma membrane and its implications in HDL metabolism.