The scattering profile of the N TAILHN -XD complex was obtained as described in Materials and Methods. Analysis of the curve in the low q region with the Guinier approximation gave an R g of 32.7 6 0.7 Å . The molecular mass (MM) calculated from the forward scattering intensity I(0) is 23.5 6 2 kDa, in agreement with the value expected for a 1:1 stoichiometric complex (21.3 kDa), thus suggesting that the complex did form in solution. The high value of R g indicated that the overall structure of the N TAILHN -XD complex is not compact. The distances distribution function inferred from the scattering curve of the N TAILHN -XD complex exhibits a maximum at 20 Å , with a shoulder at about 30 Å and a long tail up to 146 Å , typical of an elongated object (see Fig. 2A ). The bump most probably corresponds to the intramolecular distances within the globular portion of the complex (see below), while the tail indicates that N TAIL possesses regions with an extended conformation.