The glycoprotein, gp120, covers the surface of the HIV virus. It plays an essential role in HIV infection, because gp120 binds to receptor sites in the body, as the first step of viral invasion into host cells. Since the virus cannot proliferate without entering host cells, a major thrust of research directed at curing AIDS has focused on stopping the initial cellular invasion. A strategy to destroy the virus prior to cellular invasion has not yet been achieved, because the carbohydrates on gp120 present a barrier to antibody attack. While the glycans present on the HIV virus have been identified as the primary reason that the virus avoids destruction by the immune system, little can be done to overcome the virus's defense mechanism, its glycan shield, because not enough is known about the carbohydrates present at each of the 24 glycosylation sites on the protein. We have identified critical information needed to better understand the virus's defense mechanism: 1) What is the carbohydrate composition at each glycosylation site? 2) What is the topology (the degree of branching) at these sites? 3) Where is there a potential for electrostatic interaction, due to negatively charged carbohydrates, on the surface of the protein? To answer these questions, we will probe glycopeptide structure using advanced mass spectrometric instrumentation, including a MALDI TOF-TOF mass spectrometer and Linear Trap-Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (LTQ-FTMS). We are developing new mass spectrometric methods to probe the location of negatively charged glycoforms within gp120, as well as methods to identify glycopeptide topology. Furthermore, by comparing how each of these factors change from different gp120 isolates, we can determine which portions of the glycoprotein show the least propensity for variation in the carbohydrate content, among the different gp120 isolates. This information will directly impact both structure-function models of gp120, as well as provide direction for future vaccine targets. Collaborators on the project include Dr. George Bousfield, a glycobiologist from Wichita State University, and Drs. Mike Alterman and Todd Williams, Co-Directors of the University of Kansas Proteomics Facility.
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