The counterreceptor binding site of human CD2 exhibits an extended surface patch with multiple conformations fluctuating with millisecond to microsecond motions

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Abstract
We have used 15N NMR relaxation experiments to probe, for the glycosylated human CD2 adhesion domain, the overall molecular motion, as well as very fast nanosecond-picosecond (ns-ps) and slow millisecond-microsecond (ms-/xs) internal motions. Using a novel analysis method that considers all residues, we obtained a correlation time for the overall motion of 9.5 ± 0.3 ns. Surprisingly, we found a large contiguous patch of residues in the counterreceptor (CD58) binding site of human CD2 exhibiting slow conformational exchange motions (ms-ytis). On the other hand, almost none of the residues of the CD58 binding site display fast (ns-ps) internal motions of amplitudes larger than what is seen for well-ordered regions of the structure. Residues close to the N-glycosylation site, and the first N-acetylglucosamine of the high mannose glycan are as rigid as the protein core. Residues conserved in the immunoglobulin superfamily V-set domain are generally very rigid.