DNA-Based Vaccination with Polycistronic Expression Plasmids

DNA-based vaccination is a potent technique to prime cellular (T-cell mediated) immune responses (reviewed in 1). Many details of the priming of T-cell precursors by antigen translated from injected expression plasmid DNA are unknown. The relevant cell that is transfected in situ after DNA vaccination and that can process and present the protein in an immunogenic form has not yet been identified. Alternatively, the transfected cell may initiate 'cross-priming' in vivo by transferring processed antigen to a professional antigen-presenting cell (APC).

Relative functions of the alpha and beta subunits of the proteasome activator, PA28

PA28 is a 180,000-dalton protein that activates hydrolysis of small nonubiquitinated peptides by the 20 S proteasome. PA28 is composed of two homologous subunits, alpha and beta, arranged in alternating positions in a ring-shaped oligomer with a likely stoichiometry of (alphabeta)3. Our previous work demonstrated that the carboxyl terminus of the alpha subunit was necessary for PA28 to bind to and activate the proteasome. The goals of this work were to define the exact structural basis for this effect and to determine the relative roles of the alpha and beta subunits in proteasome activation. Each subunit and various mutants of the alpha subunit were expressed in Escherichia coli and purified. PA28alpha stimulated the proteasome, but had a much greater Kact than native heteromeric PA28. In contrast, PA28beta was unable to stimulate the proteasome. Mutants of the alpha subunit in which the carboxyl-terminal tyrosine residue was deleted or substituted with charged amino acids could neither bind to nor activate the proteasome. However, substitution of the carboxyl-terminal tyrosine with other amino acids resulted in proteins which could stimulate the proteasome to various extents. Tryptophan mutants stimulated the proteasome as well as did native PA28, whereas serine or phenylalanine mutants stimulated the proteasome much poorer than did wild type PA28alpha. Deletion of the "KEKE" motif, a 28-amino acid domain near the amino terminus of PA28alpha, had no effect on proteasome stimulatory activity. Hetero-oligomeric PA28 proteins were reconstituted from isolated wild type and mutant subunits. PA28 reconstituted from wild type subunits had structural and functional properties that were indistinguishable from those of the native hetero-oligomeric protein. PA28 molecules reconstituted from inactive alpha subunits and wild type beta subunits remained inactive. However, PA28 molecules reconstituted from suboptimally active alpha mutants and wild type beta subunits had the same activity as native heteromeric PA28. These results indicate that the beta subunit modulates PA28 activity, perhaps by influencing the affinity of PA28 for the proteasome.

A model for the quaternary structure of the proteasome activator PA28

PA28 is a protein activator of the 20S proteasome. It has a native molecular weight of approximately 200,000 and is composed of six 28,000-dalton subunits arranged in a ring-shaped complex. Purified preparations of PA28 contain two polypeptides, alpha and beta, which are about 50% identical in primary structure. It has been unclear whether native PA28 consists of two distinct homohexameric proteins or of a single protein containing both alpha and beta subunits. To distinguish between these possibilities, we prepared antibodies that reacted specifically with either the alpha or beta subunit and used these subunit-specific antibodies in two types of experiments designed to elucidate PA28 quaternary structure. In the first experiment, the alpha and beta subunits were completely co-immunoprecipitated by each subunit-specific antibody, indicating that both subunits were part of a single protein complex. In the second experiment, PA28 was chemically cross-linked using bis(sulfosuccinimidyl)suberate. When the cross-linked products were immunoblotted after SDS-polyacrylamide gel electrophoresis, indistinguishable patterns were obtained with each subunit-specific antibody. These results confirm that the alpha and beta subunits were part of the same protein complex. The pattern of cross-linked products also provided insight as to the relative abundance and arrangement of the subunits within the PA28 complex and indicated that the ring-shaped PA28 hexamer may be composed of alternating alpha and beta subunits with a stoichiometry of (alphabeta)3. PA28 was inactivated by treatment with carboxypeptidase Y, which cleaved Tyr and Ile residues from the carboxyl terminus of the alpha subunit but had very little effect on the beta subunit. This selective and limited proteolysis prevented binding of both alpha and beta subunits to the proteasome and therefore provides additional evidence of the heterodimeric nature of PA28. These results indicate that a short carboxyl-terminal sequence of the alpha subunit is critical for binding of native PA28 to the proteasome. To learn about the relative functions of the alpha and beta subunits, PA28alpha was expressed in Escherichia coli and purified to homogeneity. Purified PA28alpha stimulated proteasome activity but required 5-10-fold greater concentrations than the heterodimeric PA28 to achieve a given level of activity. These results suggest that the heterodimeric structure of PA28 is required for maximal proteasome activation.

[Ubiquitin-dependent degradation and modification of proteins]

A large part of cellular proteins is in a dynamic state of turnover. Protein breakdown is responsible for essential cellular functions like modulation of key enzyme levels or removal of abnormal proteins. A major pathway for this selective proteolysis is mediated by the ubiquitin system, in which proteins are committed to degradation by their ligation to ubiquitin, a highly conserved 76 amino acid polypeptide. Recent evidence indicates that ubiquitination serves other functions besides marking proteins for destruction. As originally described for histones, the activities of several cellular proteins are reversibly regulated by ubiquitination and a successive de-ubiquitination step mediated by the activity of one or more isopeptidases.

Ubiquitin in a lower plant. Characterization of ubiquitin-encoding DNA and RNA from Chlamydomonas reinhardii

A detailed characterization of Chlamydomonas reinhardii cDNAs encoding ubiquitin 52-amino-acid fusion proteins is presented in this study. While two cDNAs (designated UBI1 and UBI3) encode the complete ubiquitin extension protein, the third one (UBI2) lacks a minor part of the 5' region as well as a poly(A) tail. Differences between UBI1 and UBI3 are observed in the length of the poly(A) domain (13 versus 46 adenines) and in the lack of three nucleotides at the 3' noncoding region of UBI3. According to Northern blot experiments using UBI1 as a homologous probe, at least six members of the C. reinhardii ubiquitin gene family are transcriptionally active at regular conditions. During application of severe stress (heat shock in light and darkness, and photoinhibition), the transcription of the UBI1 mRNA substantially decreases. This effect is most drastically induced by application of heat shock to illuminated cells.

Ubiquitin-encoding mRNA and mRNA recognized by genes encoding ubiquitin-conjugating enzymes are differentially expressed in division-synchronized cultures of Chlamydomonas reinhardtii

Cells of Chlamydomonas reinhardtii were synchronized by a light/dark illumination cycle of 14:10 h. All cells divided within the first 2 h of the dark period, the synchronization index was calculated as 0.916. RNA was isolated every 2 h and hybridized to 32P-labeled probes encoding (i) ubiquitin from Chlamydomonas reinhardtii (UBM) and (ii) two different ubiquitin-conjugating enzymes from Saccharomyces cerevisiae (UBC2 and UBC3). Sequences with homology to yeast UBC2 and UBC3, which are required for sporulation/DNA repair and G1/S transition in yeast, respectively, were detected in C. reinhardtii. In the algae, the relative abundance of transcripts encoding ubiquitin fusion proteins and UBC2 homologues is most prominent at the end of the light phase and throughout the dark. The highest amount of a putative polyubiquitin encoding transcript was detected during the dark phase of the synchronized culture. A high amount of this transcript is also present during the 8th hour of the light phase which may imply that the transcription of polyubiquitin gene is not only restricted to stress conditions in C. reinhardtii. The relative abundance of transcripts with homology to UBC3 is most pronounced within the light period corresponding to G1 and S phases of the C. reinhardtii cell cycle.