Kinetic rates of antibody binding correlate with neutralization sensitivity of variant simian immunodeficiency virus strains.

Increasing evidence suggests that an effective AIDS vaccine will need to elicit both broadly reactive humoral and cellular immune responses. Potent and cross-reactive neutralization of simian immunodeficiency virus (SIV) and human immunodeficiency virus type 1 (HIV-1) by polyclonal and monoclonal antibodies is well documented. However, the mechanisms of antibody-mediated neutralization have not been defined. The current study was designed to determine whether the specificity and quantitative properties of antibody binding to SIV envelope proteins correlate with neutralization. Using a panel of rhesus monoclonal antibodies previously characterized for their ability to bind and neutralize variant SIVs, we compared the kinetic rates and affinity of antibody binding to soluble envelope trimers by using surface plasmon resonance. We identified significant differences in the kinetic rates but not the affinity of monoclonal antibody binding to the neutralization-sensitive SIV/17E-CL and neutralization-resistant SIVmac239 envelope proteins that correlated with the neutralization sensitivities of the corresponding virus strains. These results suggest for the first time that neutralization resistance may be related to quantitative differences in the rates but not the affinity of the antibody-envelope interaction and may provide one mechanism for the inherent resistance of SIVmac239 to neutralization in vitro. Further, we provide evidence that factors in addition to antibody binding, such as epitope specificity, contribute to the mechanisms of neutralization of SIV/17E-CL in vitro. This study will impact the method by which HIV/SIV vaccines are evaluated and will influence the design of candidate AIDS vaccines capable of eliciting effective neutralizing antibody responses.

Binding of polyubiquitin chains to ubiquitin-associated (UBA) domains of HHR23A.

Ubiquitin-associated (UBA) domains are small protein domains that occur in the context of larger proteins and are likely to function as inter- and intramolecular communication elements in ubiquitin/polyubiquitin signaling. Although monoubiquitin/UBA complexes are well characterized, much less is known about UBA/polyubiquitin complexes, even though polyubiquitin chains are believed to be biologically relevant ligands of many UBA domain proteins. Here, we report the results of a quantitative study of the interaction of K48-linked polyubiquitin chains with UBA domains of the DNA repair/proteolysis protein HHR23A, using surface plasmon resonance and other approaches. We present evidence that the UBL domain of HHR23A negatively regulates polyubiquitin/UBA interactions and identify leucine 8 of ubiquitin as an important determinant of chain recognition. A striking relationship between binding affinity and chain length suggests that maximum affinity is associated with a conformational feature that is fully formed in chains of n = 4-6 and can be recognized by a single UBA domain of HHR23A. Our findings provide new insights into polyubiquitin chain recognition and set the stage for future structural investigations of UBA/polyubiquitin complexes.