Dynamic light-scattering analysis of full-length human RPA14/32 dimer: purification, crystallization and self-association

Replication protein A (RPA) is a single-stranded DNA-binding protein involved in all aspects of eukaryotic DNA metabolism. A soluble heterodimeric form of RPA is composed of 14 and 32 kDa subunits (RPA14/32). Dynamic light-scattering (DLS) analysis was used to improve the purification, stabilization and crystallization of RPA14/32. Increasing the concentration of reducing agent in the last stage of purification diminished the size of a secondary peak in the anion-exchange chromatograph and promoted a single species in solution. This resulted in decreased polydispersity in the purified protein and enhanced the crystallization time from 9–­12 months to 6 d. With this homogeneous preparation, the reversible association of RPA14/32 into a dimer of dimers was demonstrated by DLS. Four different crystal forms of RPA14/32 were obtained for structure determination and complete diffraction data were collected using synchrotron radiation for three of them. Data to 2.4 Å resolution was collected from hexagonal crystals (P32 or P31; a = b = 63.0, c = 272.6 Å) and to 2.2 and 1.9 Å resolution from two orthorhombic crystal forms (both P212121; form I, a = 61.4, b = 75.2, c = 131.6 Å; form II, a = 81.8, b = 140.4, c = 173.1 Å).