Introduction

Abstract

All cells secrete a diversity of macromolecules to modify their environment or to protect themselves. On the other hand, there is the necessity to replace membrane proteins and lipids that are being constantly degraded in compartments of the secretory and endocytic pathways. Therefore, eukaryotic cells synthesize proteins for either export (secretion) or delivery to the secretory and endocytic compartments and to the plasma membrane (PM) for replacement of degraded proteins and lipids. The synthesis is carried out by ribosomes attached to the cytosolic surface of the endoplasmic reticulum (ER). The synthesis of most of cellular lipids and all fatty acids also occurs in the smooth ER. During or after the synthesis, the polypeptide chains containing transmembrane domains composed of hydrophobic amino acids are inserted into the ER membrane, whereas soluble proteins are transferred into the ER lumen. After the cleavage of their leading hydrophobic signal peptides and after protein folding both the groups of proteins are transported along the secretory pathway