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Cholesterol and Steroid Synthesizing Smooth Endoplasmic Reticulum of Adrenocortical Cells Contains High Levels of Proteins Associated with the Translocation Channel

Department of Cell Biology and Kaplan Cancer Center (V.H.B., A.S., G.K.), New York University School of Medicine, New York, New York 10016; Cellular Biochemistry and Biophysics Program (K.v.L.), Memorial Sloan-Kettering Cancer Center, New York, New York 10021; and Department of Pathology (B.L.), Columbia University, New York, New York 10027

Address all correspondence and requests for reprints to: Virginia H. Black, Department of Cell Biology, New York University School of Medicine, 550 First Avenue, New York, New York 10016. E-mail: blackv01{at}med.nyu.edu.

Steroid-secreting cells are characterized by abundant smooth endoplasmic reticulum whose membranes contain many enzymes involved in sterol and steroid synthesis. Yet they have relatively little morphologically identifiable rough endoplasmic reticulum, presumably required for synthesis and maintenance of the smooth membranes. In this study, we demonstrate that adrenal smooth microsomal subfractions enriched in smooth endoplasmic reticulum membranes contain high levels of translocation apparatus and oligosaccharyltransferase complex proteins, previously thought confined to rough endoplasmic reticulum. We further demonstrate that these smooth microsomal subfractions are capable of effecting cotranslational translocation, signal peptide cleavage, and N-glycosylation of newly synthesized polypeptides. This shifts the paradigm for distinction between smooth and rough endoplasmic reticulum. Confocal microscopy revealed the proteins to be distributed throughout the abundant tubular endoplasmic reticulum in these cells, which is predominantly smooth surfaced. We hypothesize that the broadly distributed translocon and oligosaccharyltransferase proteins participate in local synthesis and/or quality control of membrane proteins involved in cholesterol and steroid metabolism in a sterol-dependent and hormonally regulated manner.