FIG. 2.

Classification of cleavable N-terminal signal peptides. On the basis of SPase cleavage sites and the export pathways via which the preproteins are exported, predicted signal peptides (129) were divided into five distinct classes: twin-arginine (RR/KR) signal peptides, secretory (Sec-type) signal peptides, lipoprotein signal peptides, pseudopilin-like signal peptides, and bacteriocin and pheromone signal peptides. The export pathways via which the preproteins are exported and the SPases responsible for their cleavage are indicated. Most signal peptides have a tripartite structure: a positively charged N-domain (N), containing lysine and/or arginine residues (indicated by +), a hydrophobic H-domain (H, indicated by a gray box), and a C-domain (C) that specifies the cleavage site for their specific SPase. The length of the signal peptides and their subdomains is drawn to the same scale. Furthermore, helix-breaking residues, mostly glycine or proline (G/P), in the H-domain of Sec-type signal peptides are indicated. These residues are, respectively, thought to facilitate loopwise membrane insertion and cleavage by SPase I (129). Finally, where appropriate, the most frequently occurring first amino acid of the mature protein (+1) is indicated.