Role of rough and smooth microsomes in the biosynthesis of microsomal membranes.

Abstract

When a single intravenous injection of radioactive leucine was given to the rat, the labeled molecules of two microsomal enzymes in the liver, NADPH-cytochrome c reductase and cytochrome b5, appeared first in the rough microsomal membrane and subsequently in the smooth. This result is compatible with the idea that these enzymes are synthesized by the membrane-bound ribosomes in the rough region of the endoplasmic reticulum, and the newly formed enzyme molecules are then incorporated into the membrane nearest to the site of their synthesis. The distribution of the labeled enzyme molecules between rough and smooth microsomal membranes became equal at about 2 hr after the labeling and remained the same afterwards. This suggests the reversible transfer in vivo of the new enzyme molecules between these two morphologically-different portions of the endoplasmic reticulum. Examination of the total microsomal protein revealed, however, no significant difference in the distribution of new molecules between rough and smooth microsomes. In the process of the incorporation of newly synthesized molecules into the membrane, the majority of the microsomal membrane proteins thus behaves differently from the two specific enzymes examined in the present study.