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

We have previously shown that three high density lipoproteins (HDL)-binding proteins in liver, of 90, 110 and 180 kDa, are structurally related. In th is study, these proteins are identified as gp96/GRP94. This protein is know n to occur as a homodimer and has a dual subcellular localization: ii: is b oth an endoplasmic reticulum resident protein, where it is supposed to act as a chaperonin, and a plasma membrane protein, whose significance is unkno wn. In ultrastructural studies the plasma membrane localization of the homo dimeric form was verified. The 90-kDa protein was abundantly present at the membranes of the endosomal/lysosomal vesicles as well as at the apical hep atocyte membranes, comprising the bile canaliculi. The monomeric protein is scarcely present at the basolateral membrane of the hepatocytes, but could be demonstrated in coated pits, suggesting involvement in receptor-mediate d endocytosis. Labeling of the endoplasmic reticulum was virtually absent. Gp96/GRP94 was transiently expressed in COS-I cells. However, the expressed protein was exclusively localized in the endoplasmic reticulum. Transfecti on with constructs in which the C-terminal KDEL sequence had been deleted, resulted in plasma membrane localized expression of protein, but only in an extremely low percentage of cells. In order to evaluate the HDL-binding ca pacities of this protein, stably transfected cells were generated, using se veral cell types. It appeared to be difficult to obtain a prolonged high le vel expression of gp96. In these cases, however, a marked increase of E-IDL -binding activity compared with the control cells could be observed. (C) 19 99 Elsevier Science B.V. All rights reserved.