Summary of key interactions in 'atypical' WD-repeats 6 and 7 of RACK1, illustrated with the structure of yeast Asc1p (PDB: 3FRX). RACK1 proteins are characterised by a significant sequence extension between blades 6 and 7, leading to a knob-like protrusion from the upper face of the propeller. The P0 Asp in blade 7 forms a salt bridge to an Arg at the P+4 position and this is packed against a Tyr (or in some orthologues Phe) in the P-2 position. The P0 Asp is absent in blade 6. In principle, these features together with the absence of the GH signature dipeptide on the loops between blades 5 and 6 and between blades 6 and 7 may facilitate structural transitions in this region of the protein that are important for function.
Adams et al. Cell Communication and Signaling 2011 9:22 doi:10.1186/1478-811X-9-22