A PROTEIN-BINDING SPECIFICALLY TO THE IGG2B SWITCH REGION

The Abelson-virus-transformed mouse pre-B-cell line 18-81 switches alm ost exclusively from mu to gamma 2b. From nuclear extracts of this cel l line, we have isolated a factor that specifically binds to S-gamma 2 b. After an eight-step purification scheme, in which different types o f DNA-affinity chromatography were used as key elements, we obtained a preparation with two narrowly spaced bands at approximately 69 kD on a silver-stained SDS gel. Binding specificity of main-peak fractions o f affinity-purified proteins was analyzed by gel shift assays, in whic h S-gamma 2b, but not S-mu, competes. The results are consistent with this factor being part of the switch recombinase.