Participants in the interaction between protein SRP72 and the SRP RNA. a. Wildtype and mutant human SRP72 polypeptide fragments used for investigating their potential to interact with the 5e SRP RNA. Their names are given on the left. Amino acid residue numbering is according to the full-length human SRP72. Indicated are the mutated residues (underlined) and their effects on RNA-binding are shown by the colored dots. Green is for binding activities similar to the 72 h or 72 k fragments, red for no or weak binding, and pink for intermediate activities. The gray circle indicates that mutant polypeptide 72 i was prone to degradation. Potential cutting sites for Chymotrypsin (CT) are shown on top. The RNA binding region with its PDPXRWLPXXER motif (boxed) is indicated by the green brace. b. Outline of the human SRP. The RNA helices in the secondary structure (gray) are numbered from two to eight. The 5' and 3' ends and the two SRP domains are indicated, and the SRP proteins (circles and ovals) are positioned accordingly. The location of the 5e motif is shown in green as are the residue positions 100 and 240 of a neighboring loop. c. Predicted secondary structure of the 5e RNA. Numbering is according to full-length human SRP RNA excluding the helix-closing tetraloop (gray). The 11-nucleotide 5e motif is shown boxed.