Identification of the cellular proteins whose expression is regulated during the cell cycle in normal cells is essential for understanding the mechanisms involved in the control of cell proliferation. A nuclear protein called cyclin of relative molecular mass 36,000 (Mr 36K), whose synthesis correlates with the proliferative state of the cell, has been identified in several cell types of human, mouse, hamster and avian origin. The rate of cyclin synthesis is very low in quiescent cells and increases several fold after serum stimulation shortly before DNA synthesis. Immunofluorescence and autoradiography studies have shown that the nuclear staining patterns of cyclin during S phase have a sequential order of appearance and a clear correlation can be found between DNA synthesis and cyclin positive nuclei. The proliferating cell nuclear antigen (PCNA) and cyclin have many common properties and it has been shown that these two are identical. Recently a protein which is required by DNA polymerase-delta for its catalytic activity with templates having low primer/template ratios has been isolated from calf thymus. We report here that cyclin and the auxiliary protein of DNA polymerase-delta are identical.

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...

Trypanosoma brucei survives in the mammalian blood-stream by regularly changing its variant surface glycoprotein (VSG) coat. The active VSG gene is located in a telomeric expression site, and coat switching occurs either by replacing the transcribed VSG gene or by changing the expression site that is active. To determine whether VSG expression site control requires promoter-specific sequences, we replaced the active VSG expression site promoter in bloodstream-form T. brucei with a ribosomal DNA (rDNA) promoter. These transformants were fully infective in laboratory animals, and the rDNA promoter, which is normally constitutively active, was efficiently inactivated and reactivated in the context of the VSG gene expression site. As there is no sequence similarity between the VSG expression site promoter and the rDNA promoter, VSG expression site control does not involve sequences specific to the VSG expression site promoter. We conclude that an epigenetic mechanism, such as telomeric silencing, is involved in VSG expression site control in bloodstream-form T. brucei.

The survival of Trypanosoma brucei, the causative agent of Sleeping Sickness and Nagana, is facilitated by the expression of a dense surface coat of glycosylphosphatidylinositol (GPI)-anchored proteins in both its mammalian and tsetse fly hosts. We have characterized T. brucei GPI8, the gene encoding the catalytic subunit of the GPI:protein transamidase complex that adds preformed GPI anchors onto nascent polypeptides. Deletion of GPI8 (to give ⌬gpi8) resulted in the absence of GPI-anchored proteins from the cell surface of procyclic form trypanosomes and accumulation of a pool of non-protein-linked GPI molecules, some of which are surface located. Procyclic ⌬gpi8, while viable in culture, were unable to establish infections in the tsetse midgut, confirming that GPI-anchored proteins are essential for insect-parasite interactions. Applying specific inducible GPI8 RNAi with bloodstream form parasites resulted in accumulation of unanchored variant surface glycoprotein and cell death with a defined multinuclear, multikinetoplast, and multiflagellar phenotype indicative of a block in cytokinesis. These data show that GPI-anchored proteins are essential for the viability of bloodstream form trypanosomes even in the absence of immune challenge and imply that GPI8 is important for proper cell cycle progression. INTRODUCTIONTrypanosoma brucei is the heteroxenous, hemoflagellate protozoan parasite responsible for Sleeping Sickness in humans and Nagana in domestic animals in the tsetse belt of subSaharan Africa. All life cycle stages of the parasite utilize glycosylphosphatidylinositol (GPI) anchors as the predominant method for attaching proteins to their plasma membrane. In the mammalian host, these proteins include one subunit of the heterodimeric transferrin receptor (Schell et al., 1991), an alanine-rich protein of unknown function (Nolan et al., 2000), and the variant surface glycoprotein (VSG) (Ferguson et al., 1988) essential for evasion of the host's immune system.The bloodstream forms of T. brucei differentiate into procyclic forms once ingested by the tsetse fly vector. This differentiation involves remodeling of the surface by shedding the VSG coat and replacing it with an invariant coat of GPI-anchored proteins known as procyclins (Roditi et al., 1989). There are four types of procyclin, three bearing between 18 and 30 internal -Glu-Pro-repeats (EP1, EP2, and EP3; see Acosta-Serrano et al., 2000 for alignment), and one with a -Gly-Pro-Glu-Glu-Thr-(GPEET) repeat region . Although EP1 and EP3 both undergo N-glycosylation, EP2 and GPEET do not, although the latter is phosphorylated on the threonine residues of the repeat region (Butikofer et al., 1999). All procyclin isoforms are anchored by a GPI modified with a heterogeneous poly-Nacetyllactosamine side chain (Treumann et al., 1997), widely believed to form a glycocalyx over the cell surface. Displayed above this, both EP and GPEET procyclins are thought to adopt a rod-like conformation (Roditi et al., 1989;Treumann et al., 1997). Although the N-termini of the...

When the African trypanosome Trypanosoma brucei is taken up from mammals by a tse‐tse fly, it replaces its variant surface glycoprotein (VSG) coat by a procyclin coat. Transcription of VSG genes stops in the fly, but transcription of sequences derived from the promoter area of the VSG expression site(s) remains high. Whether this is due to continuing high activity of one promoter or to low activity of many promoters was unclear. We have used the small differences between the sequences of different expression sites to show that multiple expression site promoters are active in insect form trypanosomes. This is confirmed by the low expression of single copy marker genes introduced into the transcribed area. However, if the expression site promoter is removed from the genomic location of the expression site and inserted in the non‐transcribed spacer of the ribosomal DNA (rDNA), it is derepressed. Derepression of transcription can also be accomplished by replacing the promoter of an expression site by an rDNA promoter. We conclude that the down‐regulation of VSG gene expression site promoters in insect form trypanosomes is affected by both the DNA sequence of the promoter and the genomic context in which it resides.

The remarkable clinical success of Fc-fusion proteins has driven intense investigation for even more potent replacements. Using quality-by-design (QbD) approaches, we generated hexameric-Fc (hexa-Fc), a ~20 nm oligomeric Fc-based scaffold that we here show binds low-affinity inhibitory receptors (FcRL5, FcγRIIb, and DC-SIGN) with high avidity and specificity, whilst eliminating significant clinical limitations of monomeric Fc-fusions for vaccine and/or cancer therapies, in particular their poor ability to activate complement. Mass spectroscopy of hexa-Fc reveals high-mannose, low-sialic acid content, suggesting that interactions with these receptors are influenced by the mannose-containing Fc. Molecular dynamics (MD) simulations provides insight into the mechanisms of hexa-Fc interaction with these receptors and reveals an unexpected orientation of high-mannose glycans on the human Fc that provides greater accessibility to potential binding partners. Finally, we show that this biosynthetic nanoparticle can be engineered to enhance interactions with the human neonatal Fc receptor (FcRn) without loss of the oligomeric structure, a crucial modification for these molecules in therapy and/or vaccine strategies where a long plasma half-life is critical.

Multimeric fragment crystallizable (Fc) regions and Fc-fusion proteins are actively being explored as biomimetic replacements for IVIG therapy, which is deployed to manage many diseases and conditions but is expensive and not always efficient. The Fc region of human IgG1 (IgG1-Fc) can be engineered into multimeric structures (hexa-Fcs) that bind their cognate receptors with high avidity. The critical influence of the unique N-linked glycan attached at Asn-297 on the structure and function of IgG1-Fc is well documented; however, whether the N-linked glycan has a similarly critical role in multimeric, avidly binding Fcs, is unknown. Hexa-Fc contains two N-linked sites at Asn-77 (equivalent to Asn-297 in the Fc of IgG1) and Asn-236 (equivalent to Asn-563 in the tail piece of IgM). We report here that glycosylation at Asn-297 is critical for interactions with Fc receptors and complement and that glycosylation at Asn-563 is essential for controlling multimerization. We also found that introduction of an additional fully occupied N-linked glycosylation site at the N terminus at position 1 (equivalent to Asp-221 in the Fc of IgG1) dramatically enhances overall sialic acid content of the Fc multimers. Furthermore, replacement of Cys-575 in the IgM tail piece of multimers resulted in monomers with enhanced sialic acid content and differential receptor-binding profiles. Thus insertion of additional N-linked glycans into either the hinge or tail piece of monomers or multimers leads to molecules with enhanced sialylation that may be suitable for managing inflammation or blocking pathogen invasion.

In therapeutic applications in which the Fc of IgG is critically important, the receptor binding and functional properties of the Fc are lost after deglycosylation or removal of the unique Asn 297 N-X-(T/S) sequon. A population of Fcs bearing sialylated glycans has been identified as contributing to this functionality, and high levels of sialylation also lead to longer serum retention times advantageous for therapy. The efficacy of sialylated Fc has generated an incentive to modify the unique N-linked glycosylation site at Asn 297 , either through chemical and enzymatic methods or by mutagenesis of the Fc, that disrupts the protein–Asn 297 carbohydrate interface. In this study, we took an alternative approach by inserting or deleting N-linked attachment sites into the body of the Fc to generate a portfolio of mutants with tailored effector functions. For example, we describe mutants with enhanced binding to low-affinity inhibitory human Fcγ and glycan receptors that may be usefully incorporated into existing Ab engineering approaches to treat or vaccinate against disease. The IgG1 Fc fragments containing complex sialylated glycans attached to the N-terminal Asn 221 sequon bound influenza virus hemagglutinin and disrupted influenza A–mediated agglutination of human erythrocytes.