Identification of a Glycoprotein from Rat Liver Mitochondrial Inner Membrane and Demonstration of Its Origin in the Endoplasmic Reticulum*

Employing antisera against various subfractions of rat liver mitochondria (mitoplast, inner membrane, intermembrane, and matrix) as well as metabolically radiolabeled BRL-3A rat liver cells, we undertook a search for the presence of glycoproteins in this major cellular compartment for which little information in regard to glycoconjugates was available. Subsequent to [35S]methionine labeling of BRL-3A cells, a peptide:N-glycosidase-sensitive protein (45 kDa) was observed by SDS-polyacrylamide gel electrophoresis of the inner membrane immunoprecipitate, which was reduced to a molecular mass of 42 kDa by this enzyme. The 45-kDa protein was readily labeled with [2-3H]mannose, and indeed the radioactivity of the inner membrane immunoprecipitate was almost exclusively present in this component. Moreover, antisera directed against mitochondrial NADH-ubiquinone oxidoreductase (complex I) or F1F0-ATPase (complex V) also precipitated a 45-kDa protein from BRL-3A cell lysates as the predominant mannose-radiolabeled constituent. Endo-β-N-acetylglucosaminidase completely removed the radiolabel from this glycoprotein, and the released oligosaccharides were of the partially trimmed polymannose type (Glc1Man9GlcNAc to Man8GlcNAc). Cycloheximide as well as tunicamycin resulted in total inhibition of radiolabeling of the inner membrane glycoprotein, and moreover, pulse-chase studies employing metrizamide density gradient centrifugation demonstrated that the glycoprotein was initially present in the endoplasmic reticulum (ER) and subsequently appeared in a mitochondrial location. Early movement of the glycoprotein to the mitochondria after synthesis in the ER was also evident from the limited processing undergone by its N-linked oligosaccharides; this stood in contrast to lysosomal glycoproteins in which we noted extensive conversion to complex oligosaccharides. Our findings suggest that the 45-kDa glycoprotein migrates from ER to mitochondria by the previously observed contact sites between the two organelles. Furthermore, the presence of this glycoprotein in at least two major mitochondrial multienzyme complexes would be consistent with a role in mitochondrial translocations.