Endoplasmin is a reticuloplasmin

The location of endoplasmin in the endoplasmic reticulum was investigated by biochemical and immunoelectron microscopic analyses. The protein could be obtained in a soluble form by procedures that do not involve the use of any detergents. The soluble protein has the amino- and carboxy-terminal sequences of the intact molecule, showing that it has not been proteolysed. Application of the Triton X-114 phase-separation test does not reveal significant hydrophobicity in the molecule. Immunogold labelling studies on cells with a dilated endoplasmic reticulum (ER) lumen show that endoplasmin is uniformly distributed throughout the lumen, with no evidence of a preferential association with the membrane. These studies clearly demonstrate that endoplasmin is a luminal protein of the ER, i.e. a reticuloplasmin, and not an integral membrane protein.