Genes whose expression is growth factor regulated are likely to be important components in the mechanisms controlling cell proliferation and differentiation. With the aim of identifying some of those genes, a A cDNA library was prepared with poly(A)+ RNA from quiescent NIH 3T3 cells stimulated with serum for 4 h in the presence of cycloheximide. Differential screening of approximately 200,000 recombinant phage plaques revealed 2,540 clones that cross hybridized preferentially with [32P]cDNA derived from RNA of stimulated cells rather than with cDNA derived from nonstimulated cells. Cross hybridization of these clones identified 82 independent sequences, including c-fos and c-myc. Seventy-one clones were further studied. Analysis of the changes in transcription and mRNA levels after serum stimulation demonstrated that the kinetics and extent of the induction vary dramatically between the different genes. Cycloheximide in all cases superinduced the mRNA levels by two mechanisms, inhibiting the shutoff of transcription and prolonging the half-lives of the mRNAs. Our results showed that induction of proliferation is accompanied by the onset of a complex genetic program.A detailed knowledge of the biochemical events and the identification of the genes that are involved in the response to growth factors is currently being accumulated. A subset of these genes may play a critical role in cellular transformation. It is well established that the synthesis of new mRNA in quiescent cells is required for the cells to respond to mitogens and progress through Gl and enter the S phase (1). Furthermore, Gl seems to be a critical point of control of proliferation, where normal cells deprived of growth factors halt cycling, while transformed cells do not, suggesting that regulatory genes uncontrolled in the neoplastic phenotype are expressed during this transition. It has been estimated that in mouse fibroblasts, 3% of the mRNA species in logarithmically growing cells are absent in quiescent cells (49); thus, a set of these genes must be under the control of growth factors to allow cell proliferation. Evidence that gene expression could be controlled by the binding of a specific ligand to its receptor initially came from studies done with beta interferon (27,28) which demonstrated the transcriptional activation of two genes in human cells after interferon treatment. These findings were further supported by the interesting observation that the proto-oncogenes c-fos and c-myc present an immediate change in expression after stimulation of fibroblasts with growth factors (4,8,14,23,25,32), emphasizing also that important growth-controlling genes operate during the GO-to-Gl transition. Both genes are induced in the presence of protein synthesis inhibitors, demonstrating that their induction is a direct consequence of the growth factor-receptor interaction. Expression of c-fos is undetectable during the cell cycle (3), suggesting that its expression could be essential during the transition from GO to Gl and not for continuously growing ...

Zinc fingers are DNA‐binding domains present in several eukaryotic regulatory proteins. We have identified a mouse gene, Krox‐20, encoding a protein with three zinc fingers and whose expression is activated during G0/G1 transition in cultured cells. Serum stimulation of quiescent cells leads to rapid and transient accumulation of Krox‐20 mRNA, with kinetics similar to those of the c‐fos proto‐oncogene. The induction does not require de‐novo protein synthesis. In the mouse, Krox‐20 is expressed at low levels in tissues which contain rapidly dividing cells. These properties suggest that Krox‐20 encodes a transcription control factor, possibly involved in the modulation of cell proliferation.

Two strains of the parvovirus minute virus of mice (MVM), the immunosuppressive (MVMi) and the prototype (MVMp) strains, display disparate in vitro tropism and in vivo pathogenicity. We report the crystal structures of MVMp virus-like particles (MVMp b ) and native wild-type (wt) empty capsids (MVMp e ), determined and refined to 3.25 and 3.75 Å resolution, respectively, and their comparison to the structure of MVMi, also refined to 3.5 Å resolution in this study. A comparison of the MVMp b and MVMp e capsids showed their structures to be the same, providing structural verification that some heterologously expressed parvovirus capsids are indistinguishable from wt capsids produced in host cells. The structures of MVMi and MVMp capsids were almost identical, but local surface conformational differences clustered from symmetry-related capsid proteins at three specific domains: (i) the icosahedral fivefold axis, (ii) the "shoulder" of the protrusion at the icosahedral threefold axis, and (iii) the area surrounding the depression at the icosahedral twofold axis. The latter two domains contain important determinants of MVM in vitro tropism (residues 317 and 321) and forward mutation residues (residues 399, 460, 553, and 558) conferring fibrotropism on MVMi. Furthermore, these structural differences between the MVM strains colocalize with tropism and pathogenicity determinants mapped for other autonomous parvovirus capsids, highlighting the importance of common parvovirus capsid regions in the control of virus-host interactions.

This report describes the distribution of conventional nuclear localization sequences (NLS) and of a beta-stranded so-called nuclear localization motif (NLM) in the two proteins (VP1, 82 kDa; VP2, 63 kDa) forming the T‫1؍‬ icosahedral capsid of the parvovirus minute virus of mice (MVM) and their functions in viral biogenesis and the onset of infection. The approximately 10 VP1 molecules assembled in the MVM particle harbor in its 142-amino-acid (aa) N-terminal-specific region four clusters of basic amino acids, here called BC1 (aa 6 to 10), BC2 (aa 87 to 90), BC3 (aa 109 to 115), and BC4 (aa 126 to 130), that fit consensus NLS and an NLM placed toward the opposite end of the polypeptide (aa 670 to 680) found to be necessary for VP2 nuclear uptake. Deletions and site-directed mutations constructed in an infectious MVM plasmid showed that BC1, BC2, and NLM are cooperative nuclear transport sequences in singly expressed VP1 subunits and that they conferred nuclear targeting competence on the VP1/VP2 oligomers arising in normal infection, while BC3 and BC4 did not display nuclear transport activity. Notably, VP1 proteins mutated at BC1 and -2, and particularly with BC1 to -4 sequences deleted, induced nuclear and cytoplasmic foci of colocalizing conjugated ubiquitin that could be rescued from the ubiquitin-proteasome degradation pathway by the coexpression of VP2 and NS2 isoforms. These results suggest a role for VP2 in viral morphogenesis by assisting cytoplasmic folding of VP1/VP2 subviral complexes, which is further supported by the capacity of NLM-bearing transport-competent VP2 subunits to recruit VP1 into the nuclear capsid assembly pathway regardless of the BC composition. Instead, all four BC sequences, which are located in the interior of the capsid, were absolutely required by the incoming infectious MVM particle for the onset of infection, suggesting either an important conformational change or a disassembly of the coat for nuclear entry of a VP1-associated viral genome. Therefore, the evolutionarily conserved BC sequences and NLM domains provide complementary nuclear transport functions to distinct supramolecular complexes of capsid proteins during the autonomous parvovirus life cycle.The nuclear membrane offers a second barrier to those viruses that, upon specific cell surface recognition and internalization, need components of the replication and transcription machinery of the host cells for their multiplication. Indeed the structural components of karyophilic viruses reach the nucleus at two stages of the life cycle, first when the incoming particle delivers the genome and late in the infection during the nuclear accumulation of viral components leading to the biogenesis of the virions. As for the cellular components, the nuclear import of viral macromolecules must proceed across the central aqueous channel of the nuclear pore complex (NPC) (18, 58), a large structure with an eightfold rotational symmetry built from proteins called nucleoporins. Cytoplasmic-nuclear transport is directed by the interac...

The determinants of nuclear import in the VP-1 and VP-2 capsid proteins of the parvovirus minute virus of mice strain i (MVMi) synthesized in human fibroblasts were sought by genetic analysis in an infectious plasmid. Immunofluorescence of transfected cells revealed that the two proteins were involved in cooperative cytoplasmic interactions for nuclear cotransport. However, while VP-1 translocated regardless of extension of deletions and did not form capsid epitopes by itself, VP-2 seemed to require cytoplasmic folding and the overall conformation for nuclear transport. The sequence 528 KGKLTMRAKLR 538 was found necessary for nuclear uptake of VP-2, even though it was not sufficient to confer a nuclear localization capacity on a heterologous protein. In the icosahaedral MVMi capsid, this sequence forms the carboxy end of the amphipathic beta-strand I (␤I), and all its basic residues are contiguously positioned at the face that in the unassembled subunit would be exposed to solvent. Mutations in singly expressed VP-2 that either decrease the net basic charge of the exposed face (K530N-R534T), perturb the hydrophobicity of the opposite face (L531E), or distort the ␤I conformation (G529P) produced cytoplasmic subviral oligomers. Particle formation by ␤I mutants indicated that the basic residues clustered at one face of ␤I drive VP oligomers into the nucleus preceding and uncoupled to assembly and that the nuclear environment is required for MVMi capsid formation in the infected cell. The degree of VP-1/VP-2 transport cooperativity suggests that VP trimers are the morphogenetic intermediates translocating through the nuclear pore. The results support a model in which nuclear transport signaling preserves the VP-1/VP-2 stoichiometry necessary for efficient intranuclear assembly and in which the betastranded VP-2 nuclear localization motif contributes to the quality control of viral morphogenesis.The successful multiplication of many viruses depends on their gaining access to the transcription and replication machineries confined in the nucleus of the eukaryotic cell. Viruses of different compositions and sizes enter the nucleus during their life cycles in the form of particles, complexes, or virion subunits (28). These viruses use molecular interactions connecting to the physiological nuclear transport pathways of the cellular proteins. In the eukaryotic cell, most karyophilic polypeptides actively transported through the nuclear pore complex (NPC) (65) harbor a nuclear localization signal (NLS) necessary for nuclear import. In the so-called classical form, the NLS consists of a short sequence of basic amino acids either in a single cluster, as originally described for the simian virus 40 large T antigen (33), or in two domains, as for the nucleoplasmin bipartite nuclear targeting sequence (51). The import pathway of classical NLS-bearing proteins into the nucleus proceeds by consecutive interactions comprising importins ␣ and ␤ (14, 15; reviewed in reference 42), and additional soluble factors to dock to the cytopla...

A full-length cDNA clone for the human nuclear protein cyclin has been isolated by using polyclonal antibodies and sequenced. The sequence predicts a protein of 261 amino acids (Mr 29,261) with a high content of acidic (41, aspartic and glutamic acids) versus basic (24, lysine and arginine) amino acids. The identity of the cDNA clone was confirmed by in vitro hybrid-arrested translation of cyclin mRNA. Blot-hybridization analysis of mouse 3T3 and human MOLT-4 cell RNA revealed a mRNA species of approximately the same size as the cDNA insert. Expression of cyclin mRNA was undetectable or very low in quiescent cells, increasing after 8-10 hr of serum stimulation. Inhibition of DNA synthesis by hydroxyurea in serum-stimulated cells did not affect the increase in cyclin mRNA but inhibited 90% the expression of H3 mRNA. These results suggest that expression of cyclin and histone mRNAs are controlled by different mechanisms. A region of the cyclin sequence shows a significant homology with the putative DNA binding site of several proteins, specially with the transcriptional-regulator cAMP-binding protein of Escherichia coli, suggesting that cyclin could play a similar role in eukaryotic cells.The identification of the cellular proteins that are involved in the control of cell proliferation in normal cells is essential for understanding the mechanisms underlying growth regulation and cellular transformation. A nuclear protein, "cyclin" (Mr 36,000), whose synthesis correlates with the proliferative state of the cells, is potentially such a candidate (for reviews, see refs. 1 and 2). This protein is present in variable amounts in normal proliferating cells as well as transformed cells and tumors. It is highly conserved, as determined by onedimensional peptide mapping, and it has been identified in several cell types of human, mouse, hamster, and avian origin. The level of cyclin fluctuates during the cell cycle, with a clear increase during the S phase (3, 4). Moreover, a coordinate synthesis of cyclin and DNA has been demonstrated in serum or growth factor-induced quiescent cells (5, 6). The proliferating-cell nuclear antigen (PCNA; refs. 7-10), a human protein that shares the same properties, has been shown to be identical to cyclin (9, 11). Immunofluorescence studies of the distribution of cyclin (PCNA) during the cell cycle have revealed dramatic changes in its nuclear localization during the S phase (7,12,13). Recent studies have demonstrated that these changes are not triggered by a mechanism involving direct phosphorylation of cyclin (4) and that they depend on DNA synthesis or events during the S phase (12).To learn more about the structure and function of cyclin, we decided to isolate cDNA clones of the mRNA for cyclin. We report here the complete nucleotide sequence for human cyclin and its expression during the cell cycle. MATERIALS AND METHODSCells. Mouse 3T3 cells were routinely grown in Dulbecco's modified Eagle's medium supplemented with 5% fetal calf serum and antibiotics (penicillin, 100 units/ml...

Twenty-eight amino acid residues involved in most noncovalent interactions between trimeric protein subunits in the capsid of the parvovirus minute virus of mice were truncated individually to alanine, and the effects on capsid assembly, thermostability, and conformation were analyzed. Only seven side chains were essential for protein subunit recognition. These side chains virtually corresponded with those that either buried a large hydrophobic surface on trimer association or formed buried intertrimer hydrogen bonds or salt bridges. The seven residues are evolutionarily conserved, and they define regularly spaced spots on a thin equatorial belt surrounding each trimer. Truncation of the many side chains that were dispensable for assembly, including those participating in solvent-accessible polar interactions, did not substantially affect capsid thermostability either. However, the interfacial residues located at the base of the pores delineating the capsid five-fold axes participated in a heat-induced conformational rearrangement associated with externalization of the capsid protein N terminus, and they were needed for infectivity. Thus, at the subunit interfaces of this model virus capsid, only key residues involved in the strongest interactions are critical for assembly and stability, but additional residues fulfill other important biological roles. P rotein-protein recognition mediates many fundamental biological processes. A detailed knowledge of these processes requires the determination of the structural, energetic, and functional roles of individual amino acid residues and interactions in protein-protein interfaces. These studies have been generally undertaken by using small protein-ligand complexes or oligomeric proteins of moderate size (reviewed in ref. 1; see also refs. 2-4). In contrast, for multimeric protein complexes, such as viral capsids (5, 6) or large cellular assemblies, little is known about the specific molecular determinants of protein association and stability. Mutational studies of virus capsids, generally focused on a few specific amino acid residues, have provided important insights (7-22). However, exhaustive experimental studies on the relative importance of residues and molecular interactions in viral capsid assembly, disassembly, and͞or stability are still very limited. These studies contribute also to the understanding of protein structure-function relationships and evolution under conflictive selective constraints (22-27), and they could be exploited possibly in the design of thermostable vaccines and antiviral agents promoting capsid disassembly or interfering with assembly (23, 28-31).Many viruses, including viruses of medical or veterinary significance, have capsids of icosahedral symmetry. The icosahedral T ϭ 1 capsids of parvoviruses (32-37) are formed by 60 protein subunits that are contributed by three nonidentical polypeptide chains (VP1, VP2, and VP3). These polypeptides derive, however, from a single gene and show identical fold and core sequence (Fig. 1). In the minut...

We have analyzed the in vitro disassembly of the capsid of the minute virus of mice, and the stability of capsid chimeras carrying heterologous epitope insertions. Upon heating in a physiological buffer, empty capsids formed by 60 copies of protein VP2 underwent first a reversible conformational change with a small enthalpy change detected by fluorescence. This change was associated with, but not limited to, externalization of the VP2 N terminus. Irreversible capsid dissociation as detected by changes in fluorescence, hemagglutination activity, and electrophoretic mobility occurred at much higher temperatures. Differential scanning calorimetry in the same conditions indicated that the dissociation/denaturation transition involved a high enthalpy change and proceeded through one or more intermediates. In contrast, in the presence of 1.5 M guanidinium chloride, heat-induced disassembly fitted a two-state irreversible process. Both thermally and chemically induced dissociation/denaturation yielded a form that had lost a part of the tertiary structure, but still retained the native secondary structure. Data from chemical dissociation indicates this form may correspond to a molten globule-like monomeric state of the capsid protein. All five antigenic peptide insertions attempted in exposed loops, despite being perhaps among the least disruptive, led to defects in folding/assembly of the capsid and, in most cases, to reduced capsid stability against thermal dissociation. The results with one of the simplest viral capsids reveal a complex pathway for disassembly, and a reduction in capsid assembly and stability upon insertion of peptides, even within the most exposed capsid loops.The study of the folding, association, and disassembly of large multimeric proteins is complicated by their size, the general irreversibility of the reactions involved, and the frequent occurrence of off-pathway intermediates. However, the significant advances already made hold promise for a detailed understanding of these processes (1). Spherical virus capsids are large, multimeric proteins (2-5) and constitute attractive models for the study of the association, stability, and disassembly of very large protein complexes (for reviews see Refs. 4 and 6 -13). In addition, viral capsids are highly dynamic entities and have evolved unique structural solutions in response to the diverse, sometimes conflicting functions they must perform during the virus life cycle (4, 7, 11, 14 -16). Thus, they provide good opportunities to understand finely tuned structure-function relationships in proteins and to develop new antiviral approaches based on the inhibition of assembly or uncoating (15,17, 18).The icosahedral T ϭ 1 capsids of parvoviruses (19 -24) are formed by 60 protein subunits contributed by three nonidentical polypeptide chains that show, however, identical -fold and core sequence. VP2 is the major capsid protein and can selfassemble into empty (DNA-free) capsids (viral-like particles or VLPs).1 VP1, a minor component of natural capsids, includes...