NMR studies on the surface accessibility of the archaeal protein Sso7d by using TEMPOL and d(III)(DTPA-BMA) as paramagnetic probes

Understanding how proteins are approached by surrounding molecules is fundamental to increase our knowledge of life at atomic resolution. Here, the surface accessibility of a multifunctional small protein, the archaeal protein Sso7d from Sulfolobus solfataricus, has been investigated by using TEMPOL and Gd(III)(DTPABMA) as paramagnetic probes. The DNA binding domain of Sso7d appears very accessible both to TEMPOLand Gd(III)(DTPA-BMA). Differences in paramagnetic attenuation profiles of 1H–15N HSQC protein backbone amide correlations, observed in the presence of the latter paramagnetic probes, are consistent with the hydrogen bond acceptor capability of the N-oxyl moiety of TEMPOL to surface exposed Sso7d amide groups. By using the gadolinium complex as a paramagnetic probe a better agreement between Sso7d structural features and attenuation profile is achieved. It is interesting to note that the protein P-loop region, in spite of the high surface exposure predicted by the available protein structures, is not approached by TEMPOL and only partially by Gd(III)(DTPA-BMA).