Structure and Function of the Signal Peptide

This chapter describes the structural, as well as the functional, aspects of the signal peptide and discusses is the way by which it is able to guide protein translocation through the cytoplasmic membrane. The signal peptide is generally composed of approximately 20 amino acid residues and is able to direct protein translocation through the membrane. It appears that all information necessary to initiate protein secretion resides in the structure of the signal peptide. Many secretory proteins have been characterized, and the amino acid sequences of their signal peptides have been determined. In the prokaryotic system, the existence of the signal peptide was first shown for the major outer membrane lipoprotein. Since then, the existence of the signal peptide has been shown for many other prokaryotic secretory proteins. A comparison of the general structure of different signal peptides demonstrates that, independent of their origin, they share more characteristics than expected from such a diverse population of proteins. The chapter also discusses models for protein secretion. To assess the functional role of the signal peptide, two approaches have been taken. Both draw on the use of mutations that alter the functions of the signal peptide. The first approach is to isolate and to characterize signal peptide mutants that are defective in secreting proteins across the membrane. The second method is the directing of specific mutations in a signal peptide of a protein in question. In eukaryotes, several factors are known to interact with the signal peptide, whereas, in prokaryotes, the only component that has been clearly demonstrated to interact with the signal peptide is the signal peptidase.