Conformational Rearrangement within the Soluble Domains of the CD4 Receptor Is Ligand-specific

Ashish Ashish,Ignacio J Juncadella,Renu Garg,Christopher D Boone,Juan Anguita,Joanna K Krueger

doi:10.1074/jbc.m708325200

Abstract

Ligand binding induces shape changes within the four modular ectodomains (D1-D4) of the CD4 receptor, an important receptor in immune signaling. Small angle x-ray scattering (SAXS) on both a two-domain and a four-domain construct of the soluble CD4 (sCD4) is consistent with known crystal structures demonstrating a bilobal and a semi-extended tetralobal Z conformation in solution, respectively. Detection of conformational changes within sCD4 as a result of ligand binding was followed by SAXS on sCD4 bound to two different glycoprotein ligands: the tick saliva immunosuppressor Salp15 and the HIV-1 envelope protein gp120. Ab initio modeling of these data showed that both Salp15 and gp120 bind to the D1 domain of sCD4 and yet induce drastically different structural rearrangements. Upon binding, Salp15 primarily distorts the characteristic lobal architecture of the sCD4 without significantly altering the semi-extended shape of the sCD4 receptor. In sharp contrast, the interaction of gp120 with sCD4 induces a shape change within sCD4 that can be described as a Z-to-U bi-fold closure of the four domains across its flexible D2-D3 linker. Placement of known crystal structures within the boundaries of the SAXS-derived models suggests that the ligand-induced shape changes could be a result of conformational changes within this D2-D3 linker. Functionally, the observed shape changes in CD4 receptor causes dissociation of lymphocyte kinase from the cytoplasmic domain of Salp15-bound CD4 and facilitates an interaction between the exposed V3 loops of CD4-bound gp120 molecule to the extracellular loops of its co-receptor, a step essential for HIV-1 viral entry.

Highlights

To examine whether Salp15-induced complexes in three dimensions, we found a lobe with ϳ57 Å in alteration in the soluble domains of CD4 can eventually affect
Because In summary, these results showed that Salp15 binding to the the D1 domain is common between the two forms of sCD4 D1 domain of sCD4 induces a loss of the distinct domain archiused, it is very likely that Salp15 binds to one side of the apex of tecture of the sCD4 receptor without appreciably altering its the D1 domain
In comparison with the models solved for the unliganded doubling over can explain the observation that the lower sCD4 molecules, Salp15 binding leads to the loss of interdopart of the model shown in Fig. 10B is ϳ50 Å, twice the width of main shape of the receptor

Summary

Introduction

The SAXS data analysis and structure reconstruction of sCD4 molecules conclude that 1) in absence of any ligand, the truncated sCD4D1–D2 receptor adopts an extended bilobal shape in solution and 2) despite the flexible nature of the The black lines shown with the SAXS intensity reported that monomeric Salp15 forms 1:1 complexes with data of the mixtures in Fig. 6 (A and B) are the calculated I(q) both sCD4D1–D2 and sCD4D1–D4 in solution [4].

Results

Conclusion