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. Author manuscript; available in PMC: 2010 Sep 1.
Published in final edited form as: J Biomol NMR. 2009 Jul 31;45(1-2):85–98. doi: 10.1007/s10858-009-9340-0

Figure 1.

Figure 1

Structure of HP67 (PDB 2RJX, chain A). The N-terminal subdomain (residues 10–42) is colored in magenta; the C-terminal subdomain (residues 43–76) is brown. The side chain of His41 is depicted in pink stick format. The N-terminal subdomain consists mostly of turns and loops with regular secondary structure defined for three short helices: alpha helix h1 (residues 18–23), and 3–10 helices h2 and h3 (residues 26–28 and 35–41, respectively). The N-terminal loop (residues 10–17) contains the conserved residue Phe16 that forms part of the hydrophobic core. The V-loop (residues 22–28), so named for the sequence variability among headpiece homologues in the stretch of amino acids situated between conserved residues Leu21 and Leu29, encompasses h2 and is important for positioning residues in h1 and Leu29 in the hydrophobic core. B-factors for x-ray crystallographic structures and NMR generalized order parameters indicate that the V-loop is more mobile than other non-terminal residues in HP67; in addition, residues 26–28 have been shown to adopt either a 3–10 helix or an extended turn conformation under identical crystallization conditions (Meng and McKnight, 2009). The loop succeeding h2 also plays a role in positioning of conserved residues in the N-terminal subdomain hydrophobic core. Residues 30–33 form a β-turn that enables Val33 and Asp 34 to pack close to and interact with His41. The C-terminal subdomain consists of threeα-helices, h4, h5 and h6 defined by residues 44–51, 55–59 and 63–73, respectively. A triad of phenylalanine residues, 47, 51 and 58 comprise the hydrophobic core. The hydrophobic cores of the two subdomains are bridged by an interfacial hydrophobic cluster including Leu18, Leu42, Val50 and Leu75. Additional bridging contacts between the N-terminal loop and h4 further stabilizes the N-terminal subdomain and occludes His 41 from the solvent. Regions of HP67 involved in F-actin binding include residues 37–39, 63–65 and 70–76; most residues lie in the C-terminal subdomain near the actin binding surface with the exception of 37–39. The binding surface is comprised of a hydrophobic patch (63–64) encircled by a ring of mostly positive surface charge (37, 65, 71 and 73) that is situated above a distinct isolated of positive charge (38) (Vardar, et al., 1999, Meng, et al., 2005). Glu39 and Lys70 form a buried salt-bridge between subdomains that is required for high-affinity binding.