CHARACTERIZATION OF A MULTISUBUNIT HUMAN PROTEIN WHICH SELECTIVELY BINDS SINGLE-STRANDED D(GA)N AND D(GT)N SEQUENCE REPEATS IN DNA

A protein which selectively binds d(GA)n and d(GT)n sequence repeats i n single stranded DNA has been identified in human fibroblasts. This p rotein, designated PGB, has been purified at least 500-fold by ammoniu m sulfate precipitation followed by DEAE-Sepharose column chromatograp hy and affinity chromatography in a column of d(GA)-Sepharose. Electro phoretic mobility shift assays revealed that the PGB protein bound mos t avidly d(GA)n and d(GT)n tracts of n > 5. It also bound other G-rich DNA sequence repeats, including dG(n) tracts, with lower affinities. It did not manifest significant binding affinities to single stranded M13 DNA, or to the homopolynucleotides poly dA, poly dC and-poly dT, o r to various DNA sequence repeats which do not contain G residues, suc h as d(AC)n and d(TC)n. It did not bind double stranded d(TC)n.d(GA)n tracts or other double stranded DNA sequences. In glycerol gradient ce ntrifugation assays the d(GA)n- and the d(GT)n-binding activities cose dimented as a homogeneous protein species having an S20,w = 9.4 +/- 0. 7 and an estimated native molecular weight of 190,000 +/- 7,000. UV cr osslinking assays revealed that the protein contains 33.6 +/- 2.1 kd s ubunits which bind d(GA)n and d(GT)n sequences. However, SDS-polyacryl amide gel electrophoresis of the purified protein followed by silver s taining indicated that It may also contain other subunits that do not contact the DNA. It is proposed that binding of the PGB protein to sin gle stranded d(GA)n or d(GT)n tracts in double stranded topologically restricted DNA may stimulate strand separation and formation of triple helices or other unusual DNA structures.