Structure of the LAV6 peptide: A nucleation site for the correct receptor-induced refolding of the CD4-binding domain of HIV1 gp 120

Abstract

LAV44 and LAV15 (lymphadenopathy-associated virus) peptides of the CD4-binding region of gp 120 per se bind to the CD4 receptor (Reed and Kinzel, Biochemistry 30: 4521-4528, 1991; Lasky et al., Cell 50:975-985, 1987). Depending on the environment, the LAV peptides exhibit the ability to switch cooperatively between beta-sheet and helical conformation when solvent polarity is changed past a critical point. This property, which is dependent on the amino acid sequence LPCR, is crucial for receptor binding (Reed and Kinzel, Proc. Natl. Acad. Sci. U.S.A. 90:6761-6765, 1993). Structure determination with 2D-NMR-spectroscopy reveals that LAV6 peptide (sequence: TLPCRI) has a well-defined structure, partially exhibiting inverse gamma-turn conformation in aqueous solution. Quantitative evaluation of the NMR data discloses 90% trans-conformation for the peptide bond between leucine and proline. The psi- and phi-angles fall into the typical range for amino acids located in turns. On the other hand, the amino acid sequence C-terminal to the LPCR tetrad has been shown to fold atypically in the absence of these residues. All these results show that the short sequence of LAV6 peptide, with the central amino acids LPCR, displays a matrix-independent structure and may, therefore, act as a conformational template for forming secondary structure in the intact CD4-binding domain of gp 120.

Tetrad selectivity in polarity‐driven switch peptides: the best turn is not always the best nucleation site

“…Earlier NMR investigations on the hexapeptide T L PCRI—containing the original, effective tetrad—have shown that its structure in solution is quite rigid between the Leu and the Arg residues6. On the other hand, the mutant T N PCRI is highly flexible.…”

“…At the same time, preferential φ angles between the Leu2 and Cys 4 displayed values typical for a 3 10 ‐helix. It is thus well suited to form the seed for a series of overlapping turns that—triggered by a particular level of polarity—force the subsequent residues into a conformation that closely resembles a 3 10 ‐helix6. In fact, the existence of a 3 10 ‐helix could be verified for the 15mer in both, NMR measurements and MD simulations4, 7.…”

“…The tetrad has a high potential for forming β‐reverse turns, according to the Chou–Fasman algorithm5. NMR measurements in the corresponding hexapeptide (TLPCRI) in isolation revealed the existence of a γ‐inverse turn6. At the same time, preferential φ angles between the Leu2 and Cys 4 displayed values typical for a 3 10 ‐helix.…”

Molecular dynamics study of the proposed β‐hairpin form of the switch domain from HIV1 gp120 alone and complexed with an inhibitor of CD4 binding

“…3 There are some unexpected aspects of the MD structure that may be relevant to the mechanism of cooperative refolding in this peptide. The LPCR tetrad at the Nterminus of the peptide has been proposed to function as a nucleation site for the polarity-triggered refolding of the switch domain into a 3 10 helix; it has theoretical potential for forming a reverse turn, it adopts a well-defined turn structure in isolation that is compatible with the angles of a 3 10 helix 25 and, in its absence, no helix is formed. 3 All this would lead one to expect that the model would exhibit a turn centering on these residues.…”

“…It should be noted that in the inhibitor-complexed model the reverse turn at the N-terminus is centered on the LPCR tetrad, as expected from the 1 H-NMR data, 5,25 rather than on Ile 5 and Lys 6 as in the MD model of the free peptide. The and angles of these last now map to the extended region of the Ramachandran plot (Fig.…”