The C proteins of HeLa 40S nuclear ribonucleoprotein particles exist as anisotropic tetramers of (C1)3 C2.
The C proteins (C1 and C2) of HeLa 40S heterogeneous nuclear ribonucleoprotein particles copurify under native conditions as a stable complex with a fixed molar protein ratio (S.F. Barnett, W.M. LeStourgeon, and D.L. Friedman, J. Biochem. Biophys. Methods 16:87-97, 1988). Gel filtration chromatography and velocity sedimentation analyses of these complexes revealed a large Stokes radius (6.2 nm) and a sedimentation coefficient of 5.8S. On the basis of these values and a partial specific volume of 0.70 cm3/g based on the amino acid composition, the molecular weight of the complex was calculated to be 135,500. This corresponds well to 129,056, the sequence-determined molecular weight of a (C1)3C2 tetramer. Reversible chemical cross-linking with dithiobis(succinimidyl propionate) and analysis of cross-linked and cleaved complexes in sodium dodecyl sulfate-polyacrylamide gel electrophoresis confirmed that the C proteins exist as tetramers, most or all of which are composed of (C1)3C2. The tetramer is stable in a wide range of NaCl concentrations (0.09 to 2.0 M) and is not dissociated by 0.5% sodium deoxycholate. This stability is not the result of disulfide bonds or interactions with divalent cations. The hydrodynamic properties of highly purified C-protein tetramers are the same for C-protein complexes released from intact particles with RNase or high salt. These findings support previous studies indicating that the core particle protein stoichiometry of 40S heterogeneous nuclear ribonucleoproteins is N(3A1-3A2-1B1-1B2-3C1-1C2), where N = 3 to 4, and demonstrate that the C-protein tetramer is a fundamental structural element in these RNA-packaging complexes. The presence of at least three tetramers per 40S monoparticle, together with the highly anisotropic nature of the tetramer, suggesting that one-third of the 700-nucleotide pre-mRNA moiety packaged in monoparticles is associated through a sequence-independent mechanism with the C protein.
Barnett
S F
SF
Department of Molecular Biology, Vanderbilt University, Nashville, Tennessee 37235.
Friedman
D L
DL
LeStourgeon
W M
WM
D002352:Carrier Proteins / Q000302:isolation & purification*; D006367:HeLa Cells / Q000737:chemistry; D034482:Heterogeneous-Nuclear Ribonucleoprotein Group C; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D006801:Humans; D015394:Molecular Structure; D008970:Molecular Weight; D011487:Protein Conformation; D012261:Ribonucleoproteins / Q000302:isolation & purification*
C439788:C1 HNRNP; D002352:Carrier Proteins; C503635:HNRNPC protein, human; D034482:Heterogeneous-Nuclear Ribonucleoprotein Group C; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D012261:Ribonucleoproteins; C020723:citrate-binding transport protein