Protein HD40 , an RNA-helix destabilizing protein (Mr 40 000) is the major component of 30S heterogeneous nuclear ribonucleoprotein particles (hnRNP) from Artemia salina. The physical properties and ...the amino acid composition of HD40 are analogous to those of a group of well conserved core hnRNP proteins of higher eukaryotes. HD40 binds to and disrupts the secondary structure of single-stranded polynucleotides and forms extended nucleoprotein filaments at a stoichiometry of one protein per 12-15 nucleotides. The addition of a saturating amount of HD40 (one protein per 8 nucleotides) converts the filaments into bead-like complexes that are similar in properties and appearance to native hnRNP. To gain an understanding of the structure of hnRNP, we have used analytical ultracentrifugation and electron microscopy to investigate the structure of beaded complexes reconstituted from HD40 and poly(A)n of defined sizes. A complex containing 160 nucleotides forms a disc that is 3 nm high by 18 nm in diameter. At n less than 160 the complexes form sectors of the disc: 40 nucleotides give rise to a quarter of a disc, 80 nucleotides, half a disc, etc. At n greater than 160, the additional nucleoprotein elements may either initiate the formation of a second disc adjacent to the first or stack on top of the first disc to form a 6 nm high helix with a diameter of 18 nm. A single "bead" sediments at approximately 30S and contains an average of approximately 300 nucleotides and 1.8 turns of the helix. Native hnRNP particles from A. salina sediment at about 30S, have a diameter of 18-20 nm, and contain RNA fragments 180 to 400 nucleotides long.
The nucleic acid binding and unwinding properties of wild‐type Escherichia coli ribosomal protein S1 have been compared to those of a mutant form and a large trypsin‐resistant fragment, both reported ...recently J. Mol. Biol. 127, 41–45 (1979) and J. Biol. Chem. 254, 4309–4312 (1979). The mutant (ml‐S1) contains 77% and the fragment (S1‐F1) 66% of the polypeptide chain length (∼ 600 amino acid residues) of protein S1. The mutant is active in protein synthesis in vitro, the fragment, although retaining one or more of the functional domains of S1, is inactive in protein synthesis. We find that ml‐S1 is is almost as effective as S1 in binding to poly(rU), phage MS2 RNA and simian virus 40 (SV40) DNA, and in unfolding poly(rU) and the helical structures present in MS2 RNA and φX174 viral DNA. S1‐F1, however, binds to poly(rU) and denatured SV40 DNA, but not to MS2 RNA. It unfolds neither poly(rU), nor the residual secondary structure of MS2 RNA or φsX174 viral DNA. Thus, there appears to be a correlation between the loss in ability of S1 to unwind RNA and the loss in its ability to function in protein synthesis.
Glycine-rich core hnRNP proteins purified from wheat bind tightly to single-stranded but not to double-stranded nucleic acids with a preference for natural RNA over single-stranded DNA. Binding ...results in i) a progressive disruption of the residual secondary structure of the polynucleotide and the formation of an extended nucleoprotein filament until a protein to polynucleotide weight ratio of about 5:1 is attained. As more protein is added, this is followed by ii) the formation of globular structures along the polynucleotide chain with a concomitant reduction in the contour length of the nucleoprotein complex. These two features of the interaction--unwinding and condensation into beads--are analogous to the previously described behavior of the major glycine-rich core hnRNP protein from Artemia salina (Thomas et al. (1981) Proc. Natl. Acad. Sci. USA 78, 2888) and may represent the basic functional properties of this relatively well conserved group of nuclear proteins.
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