Studies on the role of the morphological constituents of the microsome fraction from rat liver in protein synthesis.

It is established that the major protein-synthesizing activity of the liver cell resides in the microsome fraction. This has been shown both by studies in the intact animal and also by incubation of the isolated fraction. More recently evidence has been obtained for the synthesis of serum albumin by such a preparation (Campbell & Kernot, 1962; Sargent & Campbell, 1963). Morphologically the liver microsome fraction is complex and it is necessary therefore to determine which of the various elements is responsible for the synthesis of protein. Palade & Siekevitz (1956) were the first to show that a major constituent of the microsome fraction is pieces of the roughsurfaced endoplasmic reticulum. This is thought to consist of a phospholipid membrane studded with ribosomes. Other elements are the smoothsurfaced reticulum, also associated with the Golgi zone, and free ribosomes. These three constituents are depicted in Fig. 1. It is not certain to what extent the different constituents are truly separate entities, for the free ribosomes may have been derived from the rough-surfaced endoplasmic reticulum, their detachment giving rise to the smooth-surfaced membrane. Nevertheless, elements with these characteristics may be seen in electron micrographs of the intact liver cell. If the microsome fraction of rat liver is treated with detergents such as deoxycholate or Lubrol then the membrane components are rendered soluble and the particles may be collected by centrifugation. As shown in Fig. 1 these particles may have come from the rough-surfaced endoplasmic reticulum or from the free ribosomes. Many call these particles obtained by the use of detergents 'ribosomes', but since it is not clear in what way the ribosomes have been affected by the detergent it seems preferable to refer to them as 'RNPt particles'. The view held at present is that the proteinsynthetic activity of the cell is confined to the