Silver stain analysis of purified PrPC substrate. Twelve percent SDS-PAGE showing (from L to R): (1) Page-ruler prestained protein ladder (Fermentas, Hanover, MD catalog #SM0671) (estimated molecular weights = 170, 130, 100, 72, 55, 40, 33, 24, and 17 kDa); (2) eluate from 3F4 immunoaffinity column; (3) flow through fraction of SP-Sepharose column; and (4–7) successive 2 ml eluate samples from SP-Sepharose column. All chromatographic fractions (lanes 2–7) were concentrated prior to electrophoresis by the method of Wessel and Flugge (14)