Isolation and Characterization of the a Helical Regions of Epidermal I’rekeratin*

1. Epidermal prekeratin, which has a molecular weight of 640,000 and an OL helix content of about 40%, has been subjected to a specific procedure involving limited tryptic digestion. Upon acidification, the digest yielded a highly Q! helical precipitate which contained all of the OL helix of prekeratin, and a supernatant. 2. The redissolved precipitate was separable by Sephadex G-ZOO chromatography into major a! helical Fractions I and II and minor non-a! helical Fractions III and IV. Fraction II was produced in increasing amounts by cleavage of Fraction I and was a relatively stable product of proteolysis. 3. Isolated Fraction II was 83% a! helical and exhibited a large degree of size homogeneity. It had a molecular weight of 46,400, a length of about 200 A, and was enriched in “helix-favoring” residues. It had three NH*-terminal amino acids and, in sodium dodecyl sulfate or urea solutions, dissociated into three chains which had molecular weights of 17,000, 15,200, and 15,000, and were present in the estimated ratio of 1.00:1.07:0.89. The three chains in the Fraction II molecule were shown to be arranged side-byside and to interact solely through secondary forces. 4. The material in the supernatant of the acidified digest was enriched in “helix-inhibiting” residues. 5. It was concluded that the (Y helix of prekeratin occurs in discrete regions, about 200 A long, which consist of a continuous or segmented triple a! helix. Between these regions are non-a helical regions in which “helix-inhibiting” residues are preferentially located.