ABSTRACT
Cell membranes and internal membranes protect cells and organelles by excluding components that will damage the enclosed cell or compartment, while including the components that are vital to the function of the cell or compartment. Small molecules, such as water and ions, move through ports specific for each, with some being controlled by conformational changes and/or adenosine triphosphate (ATP). Proteins cannot normally traverse membranes. Additionally, most proteins that are embedded in membranes cannot embed themselves in the correct membrane, or in the correct conformation. If these processes were left simply up to diffusion, the process would be inefficient and ineffective, and the cells would cease to exist. This creates a dilemma for transfer of proteins from one cell compartment to another, or for embedment into a membrane. Cellular systems have evolved to shuttle each protein to the proper location in the cell and to aid them in their interactions with membranes. However, most proteins need signal sequences that tag them to be positioned within the cell and traverse a membrane, or to become embedded in one. For example, proteins that are to end up on the outside surface of the cell membrane, or are to be excreted from the cell, have specific signal sequences on the amino ends, called signal peptides (Table 32.1). These are recognized by signal recognition particles (SRPs), which shuttle them to specific cell membranes (Figure 32.1). Other proteins then deliver them to the correct cell compartment or membrane for the proper functioning of that portion of the cell, usually by transporting them through specific protein channels that are embedded in the membrane (Figure 32.2). This is protein trafficking.
