Bioactivity and structural properties of chimeric analogs of the starfish SALMFamide neuropeptides S1 and S2

The starfish SALMFamide neuropeptides S1 (GFNSALMFamide) and S2 (SGPYSFNSGLTFamide) are the prototypical members of a family of neuropeptides that act as muscle relaxants in echinoderms. Comparison of the bioactivity of S1 and S2 as muscle relaxants has revealed that S2 is ten times more potent than S1. Here we investigated a structural basis for this difference in potency by comparing the bioactivity and solution conformations (using NMR and CD spectroscopy) of S1 and S2 with three chimeric analogs of these peptides. A peptide comprising S1 with the addition of S2's N-terminal tetrapeptide (Long S1 or LS1; SGPYGFNSALMFamide) was not significantly different to S1 in its bioactivity and did not exhibit concentration-dependent structuring seen with S2. An analog of S1with its penultimate residue substituted from S2 (S1(T); GFNSALTFamide) exhibited S1-like bioactivity and structure. However, an analog of S2 with its penultimate residue substituted from S1 (S2(M); SGPYSFNSGLMFamide) exhibited loss of S2-type bioactivity and structural properties. Collectively, our data indicate that the C-terminal regions of S1 and S2 are the key determinants of their differing bioactivity. However, the N-terminal region of S2 may influence its bioactivity by conferring structural stability in solution. Thus, analysis of chimeric SALMFamides has revealed how neuropeptide bioactivity is determined by a complex interplay of sequence and conformation

Structural analysis of the starfish SALMFamide neuropeptides S1 and S2: The N-terminal region of S2 facilitates self-association

The neuropeptides S1 (GFNSALMFamide) and S2 (SGPYSFNSGLTFamide), which share sequence similarity, were discovered in the starfish Asterias rubens and are prototypical members of the SALMFamide family of neuropeptides in echinoderms. SALMFamide neuropeptides act as muscle relaxants and both S1 and S2 cause relaxation of cardiac stomach and tube foot preparations in vitro but S2 is an order of magnitude more potent than S1. Here we investigated a structural basis for this difference in potency using spectroscopic techniques. Circular dichroism spectroscopy showed that S1 does not have a defined structure in aqueous solution and this was supported by 2D nuclear magnetic resonance experiments. In contrast, we found that S2 has a well-defined conformation in aqueous solution. However, the conformation of S2 was concentration dependent, with increasing concentration inducing a transition from an unstructured to a structured conformation. Interestingly, this property of S2 was not observed in an N-terminally truncated analogue of S2 (short S2 or SS2; SFNSGLTFamide). Collectively, the data obtained indicate that the N-terminal region of S2 facilitates peptide self-association at high concentrations, which may have relevance to the biosynthesis and/or bioactivity of S2 in vivo

A survey of diamagnetic probes for copper(2+) binding to the prion protein. H-1 NMR solution structure of the palladium(2+) bound single octarepeat

The prion protein (PrPC) is a copper binding cell surface glycoprotein which when misfolded causes transmissible spongiform encephalopathies. The cooperative binding of Cu2+ to an unstructured octarepeat sequence within PrPC causes profound folding of this region. The use of NMR to determine the solution structure of the octarepeat region of PrP with Cu2+ bound has been hampered by the paramagnetic nature of the Cu2+ ions. Using NMR we have investigated the binding of candidate diamagnetic replacement ions, to the octarepeat region of PrP. We show that Pd2+ forms diamagnetic complexes with the peptides HGGG, HGGGW and QPHGGGWGQ with 1 : 1 stoichiometry The H-1 NMR spectra indicate that these peptides are in slow-exchange between free and bound Pd2+ on the chemical-shift time-scale. We demonstrate that the Pd-peptide complex forms slowly with a time taken to reach half-maximal signal of 3 hours. Other candidate metal ions, Ni2+, Pt2+ and Au3+, were investigated but only the Pd2+ complexes gave resolvable H-1 NMR spectra. We have determined the solution structure of the QPHGGGWGQ-Pd 1 : 1 complex using 71 NOE distance restraints. A backbone RMSD of 0.30 angstrom was observed over residues 3 to 7 in the final ensemble. The co-ordinating ligands consist of the histidine imidazole side chain N epsilon, the amide N of the second and third glycines with possibly H2O as the fourth ligand. The co-ordination geometry differs markedly from that of the HGGGW-Cu crystal structure. This survey of potential replacement metal ions to Cu2+ provides insight into the metal specificity and co-ordination chemistry of the metal bound octarepeats

The cellular prion protein traps Alzheimer's A beta in an oligomeric form and disassembles amyloid fibers

This work was supported by a Wellcome Trust project grant (093241/Z/10/Z) and UK Biotechnology and Biological Sciences Research Council Quota studentships. The authors thank Harold Toms (Queen Mary, University of London) and the UK National Institute for Medical Research for NMR support, and Graham McPhail for assistance with TEM

Preferential Cu2+ coordination by His(96) and His(111) induces beta-sheet formation in the unstructured amyloidogenic region of the prion protein

The prion protein (PrP) is a Cu2+ binding cell surface glycoprotein that can misfold into a beta-sheet-rich conformation to cause prion diseases. The majority of copper binding studies have concentrated on the octarepeat region of PrP. However, using a range of spectroscopic techniques, we show that copper binds preferentially to an unstructured region of PrP between residues 90 and 115, outside of the octarepeat domain. Comparison of recombinant PrP with PrP-(91-115) indicates that this prion fragment is a good model for Cu2+ binding to the full-length protein. In contrast to previous reports we show that Cu2+ binds to this region of PrP with a nano-molar dissociation constant. NMR and EPR spectroscopy indicate a square-planar or square-pyramidal Cu2+ coordination utilizing histidine residues. Studies with PrP analogues show that the high affinity site requires both His(96) and His(111) as Cu2+ ligands, rather than a complex centered on His(96) as has been previously suggested. Our circular dichroism studies indicate a loss of irregular structure on copper coordination with an increase in beta-sheet conformation. It has been shown that this unstructured region, between residues 90 and 120, is vital for prion propagation and different strains of prion disease have been linked with copper binding. The role of Cu2+ in prion misfolding and disease must now be re-evaluated in the light of these findings

The Rapid Exchange of Zinc (2+) Enables Trace Levels to Profoundly Influence Amyloid-beta Misfolding and Dominates Assembly Outcomes in Cu (2+)/Zn (2+) Mixtures

This work was supported by the Biotechnology and Biological Sciences Research Council Project GrantBB/M023877/1 and Quota Studentship

Endocytosis of the tachykinin neuropeptide, neurokinin B, in astrocytes and its role in cellular copper uptake

The tachykinin neuropeptide, neurokinin B (NKB), belongs to a family of peptides having diverse roles in the brain. NKB, along with several other tachykinins, has been identified as a copper-binding peptide, however the physiological relevance of the binding is unclear. Previously, NKB was shown to limit the ability of copper to enter astrocytes and disrupt calcium homeostasis and it was thought that the peptide was sequestering the metal extracellularly. Here we use a fluorescein-labelled NKB peptide (F-NKB) to show that NKB is not retained extracellularly, but is endocytosed within 10-20 minutes after addition to the cell media. The endocytosis is not inhibited when NKB is delivered as a copper-complex, [CuII(F-NKB)2]. Endocytosis of NKB can increase intracellular copper. Comparison to cells cultured in copper free buffer indicated that apo-NKB can facilitate uptake of copper found in normal culture media. To achieve this NKB must compete with a variety of copper proteins, and we show that NKB can successfully compete with copper-binding peptides derived from the prion protein, itself associated with Cu(II) and Zn(II) metabolism. We suggest a mechanism of receptor mediated endocytosis to account for the observations