Structure and dynamics of capsid protein from retroviruses: NMR studies on HTLV-1 capsid protein

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that exhibits specific tropism for human T cells. The N-terminal domain of the retroviral capsid (CA) protein is one of the least conserved regions encoded in the genome. Surprisingly, the three-dimensional structures of the CA from different genera exhibit alpha-helical structural features that are highly conserved. We cloned, expressed, purified, and solved the structure of a N-terminal 134 amino acid fragment (CA₁₃₄) from the human T-cell leukemia virus type 1 (HTLV-1) using high resolution nuclear magnetic resonance (NMR) spectroscopy. The CA₁₃₄ helical core is structurally similar to other retroviral CA domains and the β-hairpin is conserved with the N-terminal Pro1 residue contacting Asp 54 in the helical cluster through a salt bridge. However, although the same Asp residue defines the orientation of the hairpin in both the HTLV-1 and HIV-1 CA proteins, the HTLV-1 beta-hairpin is oriented away, rather than towards, the helical core. Significant differences were also detected in the spatial orientation and helical content of the long centrally located loop connecting the helices in the core. It has been proposed that the salt bridge allows the formation of a CA-CA interface that is important for the assembly of the conical cores that are characteristic of HIV-1. As HTLV-1 forms spherical cores, the salt-bridge feature is apparently not conserved for this function although its role in determining the orientation of the β-hairpin may be critical, along with the central loop. High resolution NMR 15N relaxation data (T₁, T_1ρ and NOE) have been used to characterize the backbone dynamics of the N-terminal domain (NTD) of CA ₁₃₄. Our results show that the β-hairpin and the cyclophilin A (CyP A) binding loop are less flexible than the counterparts in the human immunodeficiency virus type 1 (HIV-1). To verify that, we mutated residue Asp54 to Ala which completely destabilized the beta-hairpin in the HTLV-1 NTD and caused localizatized conformational and dynamic changes in the region near the mutation, as indicated by the 15N relaxation analysis and high resolution NMR.