Purification of a telomere-binding protein from Physarum polycephalum

We have purified a telomere-binding protein (PPT) from the acellular slime mold Physarum polycephalum. As shown previously (Coren, J.S., Epstein, E.M. and Vogt, V.M. (1991) Mol. Cell. Biol. 11, 2282-2290), in vitro this protein binds specifically to the double stranded (TTAGGG)n repeats that are found at the telomeres of extrachromosomal ribosomal DNA from this organism, and also at telomeres of mammalian chromosomes. PPT was purified from Physarum nuclear extracts by heat treatment at 90 degrees C followed by heparin-agarose fractionation and gel filtration chromatography. The most purified fraction contained two major protein bands of 10 and 7 kDa when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In gel filtration chromatography PPT migrated with a Stokes radius of 1.6 nm. Along with the previously determined sedimentation coefficient of 1.2 S, this value implies a molecular weight of about 8000, making PPT the smallest known telomere-binding protein.