Assembly State of Proteasome Activator 28 in an Aqueous Solution as Studied by Small-Angle Neutron Scattering

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The characterization of quaternary structures of proteins in solution remains challenging, especially for those undergoing dynamic changes. Small-angle neutron scattering (SANS) is a potentially powerful method for addressing this issue with little perturbation resulting from irradiation damage. However, it is usually difficult to determine the three-dimensional (3D) structure of protein complexes at the atomic level on the basis of only SANS data. To cope with this difficulty, we developed a novel approach combining 3D homology modeling with SANS profile simulation, in which whole simulated SANS profiles were examined together with experimental SANS data. We herein demonstrate the feasibilty of our strategy using proteasome activator 28 (PA28) as a model system. PA28 is a hetero-oligomeric protein composed of homologous $\alpha$- and $\beta$-subunits. Although the crystal structure of the homoheptameric ring of $\alpha$-subunits (PA28$\alpha_{7}$) has been reported, the physiologically relevant hetero-oligomeric structure remains to be elucidated. On the basis of the PA28$\alpha_{7}$ structure, we performed homology modeling to build hypothetical quaternary structures of the PA28 hetero-oligomer. By analyzing the SANS data of a PA28 mutant lacking a mobile loop in its $\alpha$-subunit, we successfully revealed that $\alpha$- and $\beta$-subunits form heteroheptameric rings, about half of which are stacked back to back to form a double-ring structure. Thus, our SANS approach provides in-depth information on the assembly states of protein subunits in aqueous solutions.
2009-12-15

Assembly State of Proteasome Activator 28 in an Aqueous Solution as Studied by Small-Angle Neutron Scattering

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