Fig. 2

General features of Sec and Tat signal peptides. Sec and Tat signal peptides possess a similar tripartite overall structure consisting of a positively charged n-region, a central hydrophobic h-region, and a polar c-region that contains the recognition site (consensus: A-X-A) for signal peptidase (SPase; the cleavage site is indicated by an arrow). In Tat signal peptides, a characteristic amino acid consensus motif including two highly conserved arginine residues (underlined) is present at the boundary between the often significantly longer n-region and the h-region. Furthermore, the h-region of Tat signal peptides is mostly less hydrophobic than those found in Sec signal peptides and in the c-region of Tat signal peptides, frequently positively charged amino acids (the so-called Sec-avoidance motif) are present that prevent a mistargeting of Tat substrates into the Sec pathway