This group of archaeal proteins includes CRISPR type I-A cluster 1/Apern-associated protein Csa5-1 from Saccharolobus solfataricus (Csa5), the small subunit of the archaeal-specific Type-I-A Cascade complex for CRISPR-mediated antiviral immunity. This protein consists of two domains: an all alpha-helical arrangement and an insertion that shows a single helix and six beta-strands arranged as three anti-parallel two-stranded beta-sheets [1].

1: Structure of the archaeal Cascade subunit Csa5: relating the small subunits of CRISPR effector complexes. Reeks J, Graham S, Anderson L, Liu H, White MF, Naismith JH; RNA Biol. 2013;10:762-769. PMID:23846216