Amino acid composition bias of IPM anchors, solvent-accessible helices from globular proteins and TM anchors. Amino acid frequencies were normalized to UniProt amino acid composition (dashed line). The composition of IPM anchors is shown in black, of TM helices in grey and of solvent-accessible helices from globular proteins in white. IPM anchors are extracted from our final data set. Solvent accessible helices are extracted from globular soluble proteins present in the PDB (sequence similarity lower than 25%, accessibility computed by DSSP  lower than or equal to 60). TM helices are extracted from the 3D_helix set of the MPtopo database .