Figure 3.

Protein secondary structures of archaeal SRP19, SRP54 and the SRP receptor. Alpha helices are drawn as cylinders, beta conformations as rectangles. The N- and C-termini are indicated. Only the secondary structure of Archaeoglobus fulgidus SRP19 has been determined experimentally (Pakhomova et al. 2001). For SRP54, structural information was obtained by comparison with the NG region of SRP54 of Acidianus ambivalens (Montoya et al. 2000) and the human SRP54 M domain (Clemons et al. 1999). Delta sign posts indicate regions that are absent in Archaea but present in Bacteria, or Eukarya or both.