A B-CELL-SPECIFIC DNA RECOMBINATION COMPLEX

We have purified and biochemically characterized a multiprotein comple x designated SWAP. In a DNA transfer assay, SWAP preferentially recomb ines (''swaps'') sequences derived from Ig heavy chain switch regions. We identified four of the proteins in the SWAP complex: B23 (nucleoph osmin), C23 (nucleolin), poly(ADP-ribose) polymerase (PARP), and SWAP- 70. The first three are proteins known to be present in most cells. B2 3 promotes single-strand DNA reannealing and the formation of joint mo lecules in a D-loop assay between homologous, but also between S-mu an d S-gamma sequences. SWAP-70 is a novel protein of 70 kDa. Its cDNA wa s cloned and sequenced, and the protein was overexpressed in Escherich ia coli. SWAP-70 protein expression was found only in B lymphocytes th at had been induced to switch to various Ig isotypes and in switching B-cell lines. SWAP-70 is a nuclear protein, has a weak affinity for DN A, binds ATP, and forms specific, high affinity complexes with B23, C2 3, and poly(ADP-ribose) polymerase. These findings are consistent with SWAP being the long elusive ''switch recombinase'' and with SWAP-70 b eing the specific recruiting element that assembles the switch recombi nase from universal components.