A Single-stranded Nucleic Acid-binding Protein from Artemia salina

A protein that binds tightly to single-stranded but not to double-strained nucleic acids has been purified to homogeneity from a high salt wash of ribosomes from cryptobiotic Artemia salina gastrulae. The protein, designated HD40 to indicate a helix-destabilizing protein with a molecular weight of 40,000, is present in the high-salt ribosomal wash at a level of about 2 molecules per 80 S ribosome. The protein is monomeric at salt concentrations from 0.01 to 0.5 M and has an ahelix content of approximately 15%. The amino acid composition of HD40 is characterized by a high glycine content (19.5 mol a), the absence of cysteine, and the presence of the unusual amino acid dimethylarginine. The isolated protein binds preferentially to natural RNA over denatured DNA. HD40 inhibits protein synthesis directed by poly(rU) and by Artemia poly(A+) RNA in cell-free systems derived from Artemia and from wheat germ; inhibition is relieved by excess of mRNA. Single-stranded riboand deoxyribopolynucleotides are largely protected from degradation by nucleases when complexed with HD40.