The length of the helix including the well-ordered turn is 31A�� , which is very close to the thickness of the hydrophobic inner part of a lipid bilayer. This result WZ8040 EGFR/HER2 inhibitor suggests that Hrk-TM could fully span the bilayer leaving outside the N-terminal sequence that does not form part of the helix. Aromatic residues, specifically Trp, are particularly abundant in protein TM domains and are not evenly distributed throughout the domain, rather are concentrated at the edges . The role of these hydrophobic and bulky residues is believed to prevent the TM helix from slipping across the membrane. Hrk-TM contains 3 Trp and 1 Tyr. To illustrate the U0126 MEK inhibitor position of these residues in the membrane context the structure of Hrk-TM is placed on a model of a lipid bilayer on Fig. 8B. As these studies were done in micelles, the precise angle that the long axis of the TM helix forms with the normal of the lipid bilayer is not known. However, NMR data on amide 1H protection from the solvent suggest that the TM domain would be fully inserted in the membrane. Additionally, the two Arg residues at the Cterminus are unlikely buried in the hydrophobic part of the bilayer, as this would require a high energetic cost. These residues form a positively charged patch in the otherwise apolar electrostatic surface of Hrk-TM that could potentially form electrostatic interactions with the polar heads of the bilayer. These results suggest that Hrk-TM likely adopts a position close to parallel to the bilayer normal as shown in Fig. 8B, C. Summarizing, the monomeric state of Hrk-TM and its structural characteristics suggest that it functions as a membrane-anchoring device. This study reveals high-resolution structural characteristics of both the cytosolic and the TM domain of the Bcl-2 protein Hrk. The cytosolic domain is largely unstructured in solution , which indicates that Hrk shares features commonly found in intrinsically disordered proteins. Three other members of the BH3-only subfamily have been shown to be unstructured . Thus, intrinsic disorder most likely plays an important role in the function of these proteins. However, no structural characterization at atomic resolution of a natively unfolded Bcl-2 member has been reported up to date. In the case of Hrk, our study shows that out of the 59 residues in the cytosolic domain only a stretch of 25 , including the BH3 region, significantly populates the a-helical conformation.