Figure 4

The M-motif is a common structural element formed independent of the SIL consensus sequence. (A) The two known tetraspanin structures with resolved M-motif and additional 16 examples of crystal structures containing in total 20 inter-helix turn M-motifs. Proteins are listed using short names, followed by the PDB entry number and the sequence of the M-motif. Mostly, the M-motif was found in membrane proteins but also in three cytosolic ones (marked with an asterisk). Examples in which the motif connects two TMSs were highlighted (PDB number in bold). For structural illustration of the M-motifs in the high-affinity copper transporter Ctrl1, V-ATPase and alpha-soluble NSF attachment protein see Fig. S5. (B) Amino acid frequencies in the M-motifs of the proteins listed in A. With the exception of the central asparagine, there is hardly overlap with the animal SIL core consensus sequence. (C) Analysis of the dihedral angles employing a Ramachandran plot reveals alpha-helical properties in positions 1, 4 and 5, left-handed helix in position 2, and beta-strand features in position 3. As expected, neighbouring residues are essentially alpha-helical. (D) The 180° turn of the direction of the protein backbone involves three structural elements. The turn is initiated by the left-handed character adopted by position 2. Between position 2 and 4 the gap is spanned by the beta-strand character of position 3. Finally, at position 4 already alpha-helicity in the opposite direction is realized. Please note that arrows shown at position 2 and 4 look identical but indicate different angle combinations, which is due to simplification in structure presentation. The alignment was created using BioEdit²⁸ v7.0.5 (http://www.mbio.ncsu.edu/BioEdit/bioedit.html) and the illustration of residue frequency was done using Weblogo³³ 3.7.4 (https://weblogo.berkeley.edu/logo.cgi). The presentation of the dihedral angles was done using GraphPad Prism version 6.04 for Windows (www.graphpad.com) and PyMOL 2.5 (https://pymol.org/2/).