The RRM fold of RAMPs and Cas2. The RRM fold domains of Cas2 and the three major RAMP groups proposed in the text are shown in cartoon representation with their N- and C- termini indicated. In Cas7, the insertions into the core of the RRM fold are shown in a darker shade. In the RAMPs with two RRM fold domains, these are respectively labeled as N(-terminal) and C(-terminal). The distinct C-terminal domains of Cas5 and Cas6f (Csy4) are also shown. In Cas6f, the glycine-rich loop, which is embedded in a beta-hairpin in contrast to the typical helix-strand element, is colored orange. Note the "horizontal" packing of the first helix of the core RRM fold against the 4 strand sheet, which is one of the characteristic structural features of the RAMPs (apparent in Cas7, Cas6, Cas6e and Cas5). The following PDB ids were used to generate these representations: 2I0X (Cas2);_3PS0 (Cas7); 3I4H (Cas6); 1WJ9 (Cas6e/CasE); 3KG4(Cas5); 2XLJ (Cas6f/Csy4).