Protease activities during preparation and handling of nuclear particles containing hnRNA

Large-scale purification of hnRNP proteins from HeLa cells by affinity chromatography on ssDNA-cellulose

A purification procedure for proteins which bind heterogeneous nuclear RNA (hnRNP proteins) is described. The procedure, which entails standard chromatographic fractionations (single-stranded DNA cellulose, hydroxyapatite) and detection with specific antibodies, allows a large-scale preparation of these proteins and the partial separation of different polypeptides. By this method, polypeptides of higher molecular mass (53-55 kDa) can be purified, which are structurally and antigenically related to the 'canonical' hnRNP core proteins (34-43 kDa) that constitute the 40S hnRNP complexes. We also show that HeLa cells contain a protease that cleaves hnRNP core proteins to discrete smaller polypeptides of 22-28 kDa. Such protease, which has been partially purified, appears to copurify extensively with some of the hnRNP proteins.

Characterization of a messenger RNA transport protein

A cytoplasmic protein which facilitates the energy-dependent transport of mRNA from isolated nuclei to a specified medium has been further characterized, since it could have relevance to the mechanism of mRNA nucleo-cytoplasmic transport in vivo. This protein is now shown, by cDNA hybridization analysis using appropriate recombinant probes, to be obligatory for the transport of alpha 2u-globulin and albumin mRNA from male rat liver nuclei. It is concentrated in the cytoplasm. When isolated under conditions where they retain nuclear proteins, the nuclei contain less than 2% of the total mRNA transport activity. Approx. 20% is recovered in the cytosol, while the rest (80%) copurifies with the messenger ribonucleoproteins in the polyribosome fraction. The protein is eluted from the poly A-messenger ribonucleoproteins between 0.25 and 0.50 M NaCl. The activities of the cytosolic- and messenger ribonucleoprotein-derived transport proteins were mutually additive below saturation of the transport system. Further, the activities of both fractions were increased when they were fortified with the catalytic subunit of the cAMP-dependent protein kinase in the presence of ATP. On the other hand, protein kinase-induced thiophosphorylation of the protein with ATP[S] decreased transport activity. The molecular weight of the transport protein from either cell compartment as judged by molecular sieving is approx. 35,000. It has now been purified 2000-fold and requires manganese ions and serum albumin for stabilization of activity. The highly purified transport factor from the cytosol is tentatively assigned a molecular weight of 32,000 by SDS-polyacrylamide gel electrophoresis.

Maturation of a 100 kDa protein associated with preribosomes in Chinese hamster ovary cells

A 100 kDa nucleolar protein which is transitorily associated with preribosomes in the nucleoli of Chinese hamster ovary cells has been found to be specifically cleaved by a thiol protease. During an 'in vitro' incubation of nucleoli, the 100 kDa protein is processed into eight different proteins which are detected by immunoreaction with a serum raised against the 100 kDa protein. Qualitative and quantitative variations in the maturation products of the 100 kDa protein are obtained by 'in vitro' incubation of the 60S and 80S preribosomes. The 100 kDa protein has been purified to homogeneity with the protease activity still associated. The properties of the enzyme are described and its role in the maturation of preribosomes is discussed.

Detection and localization of a class of proteins immunologically related to a 100-kDa nucleolar protein

A high-molecular-mass nucleolar protein (100-kDa protein) is associated with nascent pre-rRNA and pre-ribosomes in Chinese hamster ovary cells. We have prepared antiserum against the 100-kDa protein and we have used it to study the intracellular localization and the possible processing of this protein. Serologically related proteins were detected in the nucleolus and in ribosomes. Proteins of various subcellular fractions were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis, transferred to nitrocellulose filters, reacted with serum and the protein-immunoglobulin complexes were revealed by 125I-labeled protein A. In the nucleolus, four proteins with molecular masses of 100 kDa, 95 kDa, 76 kDa and 70 kDa were thus visualized. In the ribosomes, two proteins (of 100 kDa and 76 kDa) gave a strong reaction while six others (of 70 kDa, 60 kDa, 50 kDa, 30 kDa, 21 kDa, 18 kDa) reacted slightly. By immunological precipitation of total cell extracts, we have shown that the 100-kDa protein antiserum cross-reacts with five proteins with molecular masses of 120 kDa, 100 kDa, 95 kDa, 70 kDa and 60 kDa. Specific degradation of the 100-kDa protein into similar peptides with molecular masses of 95 kDa, 76 kDa, 70 kDa, 60 kDa and 50 kDa can be achieved by incubation of isolated nucleoli or of purified 100-kDa protein in vitro. Cleavage of the protein is due to a thiol endoprotease which is tightly bound to the 100-kDa protein. Possible relations between the maturation of this preribosomal protein into ribosomal proteins and the processing of preribosomal RNA into mature ribosomal RNA are discussed.

In vitro ribosomal ribonucleoprotein transport upon nuclear expansion

The interdependence of nuclear rRNA release and nuclear size is investigated in macronuclei isolated from Tetrahymena. Nuclei are induced to contract and to expand, without any structural disintegration of the nuclear envelope, by final Ca2+/Mg2+ (3:2) concentrations of 5 and 1.5 mM, respectively. Upon expansion, the average volume of nuclei increases from 600 +/- 42 to 811 +/- 76 micron3. Concomitantly, nuclei begin to release RNA following saturation kinetics. This RNA release stops immediately upon nuclear contraction. Similar to the in vivo situation, only advanced rRNA processing products are released in the form of ribosomal precursor particles, as identified in detail by polyacrylamide gel electrophoresis and rate zonal and isopycnic density gradient centrifugation. Three particle ty9es are released having average buoyant densities of 1.495, 1.470, and 1.532 g/cm3, exhibiting average sedimentation coefficients of 62, 62, and 35 S, and containing the immediate precursor to the 25S rRNA, 26S rRNA, and 17S rRNA, respectively. Tje rRNP release if ATP independent and noncoincident with the release of endogenous nuclear Pi, though it is Be2+ sensitive. Our data are compatible with the views that nuclear expansion is the prerequisite rather than the cause for the rRNP release and that nuclear pore complex associated ATPases play only, if at all, a minor role in nucleocytoplasmic exchange of rRNP.

Characterization of pre-messenger-RNA-containing nuclear ribonucleoprotein particles from avian erythroblasts

Ribonucleoprotein particles have been isolated from duck erythroblast nuclei using a procedure designed to produce maximal cytoplasmic dispersion with minimal release of endogenous hydrolytic enzymes. The RNA extracted from the purified nuclear ribonucleoprotein fraction is shown to contain globin messenger RNA sequences at a concentration comparable to that present in total nuclear RNA. The polypeptide composition of this fraction revealed by electrophoresis in two dimensions is complex, consisting of at least 65 acidic species and 21 basic species. Several lines of evidence suggest that these are authentic components of nuclear ribonucleoprotein. The so-called 'core' proteins of nuclear ribonucleoprotein which were previously shown to migrate as a single band on low-pH urea gels, and as six bands on sodium dodecyl sulphate gels are here shown to be considerably more complex being resolved by two-dimensional electrophoresis into a group of 15 basic and 6 more and less neutral polypeptides. Isoelectric focusing of nuclear ribonucleoprotein under non-denaturing conditions suggests that these latter species are not uniformly distributed along the pre-messenger RNA molecule.

Proteinases as probes of the structure of rat liver ribonucleoprotein particles carrying heterogeneous nuclear RNA

Three proteolytic enzymes with different specificities produced essentially the same degradation pattern of the proteins of ribonucleoprotein particles. Proteins with apparent molecular weights of 144 000, 133 000 and 115 000 were hydrolyzed most readily which may indicate an exterior location in the ribonucleoprotein complex. The major particle proteins in the range of 30 000-42 000 daltons were much less susceptible to proteinases than the high molecular weight species. Proteins of 31 000, 33 000 and 42 000 daltons were resistant to proteolysis. The degradation rates of the proteins in intact ribonucleoprotein particles were considerably lower than rates obtained with the same proteins separated from the RNA. Incubation with trypsin and recentrifugation of polyparticles led to a shift in the sedimentation constant from 90 S to 0-20 S, corresponding to a decrease in the protein to RNA ratio from 4 to 1.3. No significant change in the small molecular weight RNA pattern was detected after trypsin digestion.

Rearrangements in the course of ribonuclease hydrolysis of pre-messenger ribonucleoproteins. A warning

A study of the action of ribonuclease on 30--50-S monoparticles prepared from pre-messenger ribonucleoprotein (pre-mRNA . protein) was started in order to elucidate the structure of monoparticles. A ribonucleoprotein complex containing mostly 30000--38000-Mr proteins of pI 7--9 (alpha class) persisted under conditions where other proteins (23000--110000 Mr, pI 5--8.5, beta class) were relased. An unexpected increase of sedimentation coefficient accompanied the formation of the ribonucleoprotein complex. The extent of increase varied with the initial size of the monoparticles, reaching 45% for 30-S monoparticles. The ribonucleoprotein complexes designated here as 40--45-S alpha-ribonucleoproteins were more homogeneous in size than the original monoparticles. Electron microscopic examination showed that the sedimentation shift corresponded to an increase of the actual size of the particles, not to flattening or change of shape. Therefore, the 40--45-S alpha-ribonucleoprotein is not a pre-existing unit of pre-mRNA . protein but arises from specific rearrangements probably between small alpha ribonucleoproteins formed by fragmentation of monoparticles. In addition to the 40--45-S alpha-ribonucleoproteins, large protein aggregates corresponding to 15% of the monoparticle proteins were formed upon ribonuclease hydrolysis. Their major proteins were neutral, suggesting that the aggregates might be precipitates of proteins at pH close to the pI. Ribonuclease being a widespread cellular enzyme, partial rearrangements may occur during preparation and handling of pre-mRNA . protein. It is particularly crucial to remark that the 40--45-S alpha-ribonucleoprotein which does not pre-exist might be mistaken for a pre-mRNA . protein unit.