Interactions between the gp120 and gp41 subunits of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer maintain the metastable unliganded form of the viral spike. Binding of gp120 to the receptor, CD4, changes the Env conformation to promote gp120 interaction with the second receptor, CCR5 or CXCR4. CD4 binding also induces the transformation of Env into the prehairpin intermediate, in which the gp41 heptad repeat 1 (HR1) coiled coil is assembled at the trimer axis. In nature, HIV-1 Envs must balance the requirements to maintain the noncovalent association of gp120 with gp41 and to evade the host antibody response with the need to respond to CD4 binding. Here we show that the gp41 HR1 region contributes to gp120 association with the unliganded Env trimer. Changes in particular amino acid residues in the gp41 HR1 region decreased the efficiency with which Env moved from the unliganded state. Thus, these gp41 changes decreased the sensitivity of HIV-1 to cold inactivation and ligands that require Env conformational changes to bind efficiently. Conversely, these gp41 changes increased HIV-1 sensitivity to small-molecule entry inhibitors that block Env conformational changes induced by CD4. Changes in particular gp41 HR1 amino acid residues can apparently affect the relative stability of the unliganded state and CD4-induced conformations. Thus, the gp41 HR1 region contributes to the association with gp120 and regulates Env transitions from the unliganded state to downstream conformations.IMPORTANCE The development of an efficient vaccine able to prevent HIV infection is a worldwide priority. Knowledge of the envelope glycoprotein structure and the conformational changes that occur after receptor engagement will help researchers to develop an immunogen able to elicit antibodies that block HIV-1 transmission. Here we identify residues in the HIV-1 transmembrane envelope glycoprotein that stabilize the unliganded state by modulating the transitions from the unliganded state to the CD4-bound state.

The envelope (Env) glycoproteins of human immunodeficiency virus (HIV-1) mediate viral entry and are also the primary target of neutralizing antibodies. The gp160 envelope glycoprotein precursor undergoes proteolytic cleavage in the Golgi complex to produce the gp120 exterior glycoprotein and the gp41 transmembrane glycoprotein, which remain associated non-covalently in the trimeric Env complex. Monomeric soluble gp120 has been used extensively to investigate conformational states, structure, antigenicity and immunogenicity of the HIV-1 Env glycoproteins. Expression of gp120 alone (without gp41) leads to the accumulation not only of monomeric gp120 but also an aberrant dimeric form. The gp120 dimers were sensitive to reducing agents. The formation of gp120 dimers was disrupted by a single amino acid change in the inner domain, and was reduced by removal of the V1/V2 variable loops or the N and C termini. Epitopes on the gp120 inner domain and the chemokine receptor-binding surface were altered or occluded by gp120 dimerization. Awareness of the existence and properties of gp120 dimers should assist interpretation of studies of this key viral protein.

In a graphical user interface, physical layout and abstract structure are two important aspects of a graph. This article proposes a new graph grammar formalism which integrates both the spatial and structural specification mechanisms in a single framework. This formalism is equipped with a parser that performs in polynomial time with an improved parsing complexity over its nonspatial predecessor, that is, the Reserved Graph Grammar. With the extended expressive power, the formalism is suitable for many user interface applications. The article presents its application in adaptive Web design and presentation.

The sensitivity of a neural network's output to its input perturbation is an important issue with both theoretical and practical values. In this article, we propose an approach to quantify the sensitivity of the most popular and general feedforward network: multilayer perceptron (MLP). The sensitivity measure is defined as the mathematical expectation of output deviation due to expected input deviation with respect to overall input patterns in a continuous interval. Based on the structural characteristics of the MLP, a bottom-up approach is adopted. A single neuron is considered first, and algorithms with approximately derived analytical expressions that are functions of expected input deviation are given for the computation of its sensitivity. Then another algorithm is given to compute the sensitivity of the entire MLP network. Computer simulations are used to verify the derived theoretical formulas. The agreement between theoretical and experimental results is quite good. The sensitivity measure can be used to evaluate the MLP's performance.

No abstract