The conformation and dynamics of CD4-183, a soluble fragment of CD4, are being investigated. CD4 is found on the surface of helper T-cells where it binds the antigen bearing class-II major histocompatibility complex of macrophages in an early stage of the humoral immune response. CD4 also is the T-cell binding site for the AIDS virus HIV. CD4-183 contains the first two domains of CD4 including the region in domain 1 that binds the HIV coat protein gp120. H-exchange studies have been conducted on 15N labeled samples at pH 5.8 on the DMX 750 spectrometer. A preliminary analysis of the data using peak assignments from earlier studies has confirmed previous H-exchange results for domain 2 and has extended the results to domain 1. Notably, amide proton resonances for residues 35-53, the gp120 binding site, were not observed in the first spectrum of the experiment suggesting that the amides in this region of the molecule are accessible to solvent and may be relatively flexible. Peak chemical shifts from HNCO, HNCA, HN(CO)CA, HCACO, CBCA(CO)NH, and 15N NOESY-HMQC and TOCSY-HMQC experiments have been used to test the peak assignment software CONTRAST. The results of the CONTRAST analysis of the data correlated very well with assignments made by hand. Regions of the sequence where manual assignments were tentative also scored low in the CONTRAST run and possible alternative assignments were identified. Assignments supported by multiple sequential connectivities scored high, as expected for unambiguous well documented results.
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