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The structural protein group-specific antigen (Gag) is the only viral product required for retrovirus assembly and will form virus-like particles (VLPs) in vitro in the presence of nucleic acids. The nature of the contribution of nucleic acid to particle assembly and structure is not fully understood. Further, interactions of Gag protein with nucleic acids have not been characterized in any detail. We have utilized a quartz crystal microbalance with dissipation monitoring (QCM-D) to investigate the possible assembly of Gag particles in the presence of three oligonucleotides—TGx2.5, TGx10, and TGx20 (a 5-mer, 20-mer, and 40-mer, respectively). QCM-D provides information on both mass changes, measured as a change in frequency, and structural changes measured as changes in energy dissipation.
TGx2.5, TGx10, and TGx20 were immobilized, via a biotin linker, to an amine-coupled NeutraAvidin (Pierce Chemical Co., Rockford, IL) surface. HIV Gag protein was then passed over the surfaces via a peristaltic pump. QCM-D data are summarized as follows: The frequency values decreased with time when Gag was flowed over the oligo surfaces, indicating binding of protein. A continuous decrease in dissipation and frequency values was observed during the binding of Gag protein with the longest oligonucleotide, TGx20, indicating increase in the rigidity of the oligo-protein film. This suggests the formation of intermolecular complex(es) between the bound oligo and the protein. Binding of Gag with TGx10 and TGx2.5 showed varying dissipation values with time. This indicates oligo-protein assemblies go through several structural changes during the course of binding. The control NeutraAvidin surface exhibited an increase in dissipation on binding with Gag that is typical of mass adsorption, indicating an increase in the viscoelasticity of the adsorbed film.