Inhibitory and blocking monoclonal antibody epitopes on merozoite surface protein 1 of the malaria parasite Plasmodium falciparum

Merozoite surface protein 1 (MSP-1) is a precursor to major antigens on the surface of Plasmodium spp. merozoites, which are involved in erythrocyte binding and invasion. MSP-1 is initially processed into smaller fragments; and at the time of erythrocyte invasion one of these of 42 kDa (MSP-1(42)) is subjected to a second processing, producing 33 kDa and 19 kDa fragments (MSP-1(33) and MSP-1(19)). Certain MSP-1-specific monoclonal antibodies (mAbs) react with conformational epitopes contained within the two epidermal growth factor domains that comprise MSP-1(19), and are classified as either inhibitory (inhibit processing of MSP-1(42) and erythrocyte invasion), blocking (block the binding and function of the inhibitory mAb), or neutral (neither inhibitory nor blocking). We have mapped the epitopes for inhibitory mAbs 12.8 and 12.10, and blocking mAbs such as 1E1 and 7.5 by using site-directed mutagenesis to change specific amino acid residues in MSP-1(19) and abolish antibody binding, and by using PEPSCAN to measure the reaction of the antibodies with every octapeptide within MSP-1(42). Twenty-six individual amino acid residue changes were made and the effect of each on the binding of mAbs was assessed by Western blotting and BIAcore analysis. Individual changes had either no effect, or reduced, or completely abolished the binding of individual mAbs. No two antibodies had an identical pattern of reactivity with the modified proteins. Using PEPSCAN each mAb reacted with a number of octapeptides, most of which were derived from within the first epidermal growth factor domain, although 1E1 also reacted with peptides spanning the processing site. When the single amino acid changes and the reactive peptides were mapped onto the three-dimensional structure of MSP-1(19), it was apparent that the epitopes for the mAbs could be defined more fully by using a combination of both mutagenesis and PEPSCAN than by either method alone, and differences in the fine specificity of binding for all the different antibodies could be distinguished. The incorporation of several specific amino acid changes enabled the design of proteins that bound inhibitory but not blocking antibodies. These may be suitable for the development of MSP-1-based vaccines against malaria.

Regulation of the homeodomain CCAAT displacement/cut protein function by histone acetyltransferases p300/CREB-binding protein (CBP)-associated factor and CBP

The CCAAT displacement protein/cut homologue (CDP/cut) is a divergent homeodomain protein that is highly conserved through evolution and has properties of a potent transcriptional repressor. CDP/cut contains three conserved cut-repeat domains and a conserved homeobox, each involved in directing binding specificity to unique nucleotide sequence elements. Furthermore, CDP/cut may play a role as a structural component of chromatin through its direct interaction with nucleosomal DNA and association with nuclear matrix attachment regions. CDP/cut is cell-cycle regulated through interactions with Rb, p107, specific kinases and phosphatases directing the transcriptional activity of CDP/cut on such genes encoding p21(WAF1,CIP1), c-myc, thymidine kinase, and histones. Our previous studies indicate that CDP/cut is associated with histone deacetylase activity and is associated with a corepressor complex through interactions with histone deacetylases. Here, we report the interaction of CDP/cut with CBP and p300/CREB-binding protein-associated factor (PCAF) along with the modification of CDP/cut by the histone acetyltransferase PCAF. Acetylation of CDP/cut by PCAF is directed at conserved lysine residues near the homeodomain region and regulates CDP/cut function. These observations are consistent with the ability of CDP/cut to regulate genes as a transcriptional repressor, suggesting acetylation as a mechanism that regulates CDP/cut function.

Transcriptional repression of the cystic fibrosis transmembrane conductance regulator gene, mediated by CCAAT displacement protein/cut homolog, is associated with histone deacetylation

Human cystic fibrosis transmembrane conductance regulator gene (CFTR) transcription is tightly regulated by nucleotide sequences upstream of the initiator sequences. Our studies of human CFTR transcription focus on identifying transcription factors bound to an inverted CCAAT consensus or "Y-box element." The human homeodomain CCAAT displacement protein/cut homolog (CDP/cut) can bind to the Y-box element through a cut repeat and homeobox. Analysis of stably transfected cell lines with wild-type and mutant human CFTR-directed reporter genes demonstrates that human histone acetyltransferase GCN5 and transcription factor ATF-1 can potentiate CFTR transcription through the Y-box element. We have found 1) that human CDP/cut acts as a repressor of CFTR transcription through the Y-box element by competing for the sites of transactivators hGCN5 and ATF-1; 2) that the ability of CDP/cut to repress activities of hGCN5 and ATF-1 activity is contingent on the amount of CDP/cut expression; 3) that histone acetylation may have a role in the regulation of gene transcription by altering the accessibility of the CFTR Y-box for sequence-specific transcription factors; 4) that trichostatin A, an inhibitor of histone deacetylase activity, activates transcription of CFTR through the Y-box element; 5) that the inhibition of histone deacetylase activity leads to an alteration of local chromatin structure requiring an intact Y-box sequence in CFTR; 6) that immunocomplexes of CDP/cut possess an associated histone deacetylase activity; 7) that the carboxyl region of CDP/cut, responsible for the transcriptional repressor function, interacts with the histone deacetylase, HDAC1. We propose that CFTR transcription may be regulated through interactions with factors directing the modification of chromatin and requires the conservation of the inverted CCAAT (Y-box) element of the CFTR promoter.

Sequence-specific DNA binding of individual cut repeats of the human CCAAT displacement/cut homeodomain protein

CCAAT displacement protein (CDP), a nuclear protein of 180-190 kDa, contains a triplicated motif, the cut domain, similar (80-90% conserved) to three repeats of 60-65 amino acids first identified in Drosophila cut, a homeo-domain protein involved in cell-fate decisions in development. Cut repeats bind DNA and exhibit subtle differences in target-site recognition. DNA sequences specifically bound by cut repeats were isolated by PCR-mediated DNA target-site selection. Sequences selected for cut repeat 2 and 3 (CR2 and CR3) binding are A+T-rich and favor an ATA motif with similar, but not identical, flanking base preferences. CR2 and CR3 discriminate among similar target sequences. CR1, which is more divergent from CR2 and CR3, displays the most restricted pattern of DNA sequence recognition. Methylation interference analysis demonstrates different protein-DNA contacts for CR1 and CR3 binding to a target sequence. Thus, CDP/cut is a complex protein whose DNA-binding properties reflect the combinatorial interaction of four domains (three cut repeats and one homeodomain) with target DNA sequences.

Alternate translation initiation on hepatitis B virus X mRNA produces multiple polypeptides that differentially transactivate class II and III promoters

The hepatitis B virus X gene encodes a transcription activator which stimulates the synthesis of RNAs from a variety of class II and III promoter elements. In this report, we present a mutational analysis which genetically demonstrates that the X gene actually encodes two, and possibly three, related polypeptides from a single mRNA using alternate translation initiation from any of three in-frame AUG codons. Genetic analysis shows that translation initiates at the 5' proximal AUG of X mRNA and produces a full-length 17-kDa X protein but in addition also likely initiates at either of two conserved, in-frame AUG codons, producing two amino-terminally truncated X proteins presumably of 8 and 6.6 kDa. Expression of mRNAs capable of encoding only one of each X protein all individually transactivate class III (RNA polymerase III)-transcribed promoters. However, class II (RNA polymerase II)-transcribed promoters displayed various requirements for the different X proteins. Expression of two X proteins, the 17- and 6.6-kDa species, was required to activate transcription of the simian virus 40 enhancer/early promoter. In contrast, activation of an NF-kappa B-dependent promoter was carried out only by mRNAs encoding the full-length 17-kDa X protein. These results indicate that the X gene encodes several related proteins that possess different transcriptional regulatory activities.

The hepatitis B virus X-gene product trans-activates both RNA polymerase II and III promoters

The transcriptional regulatory activity of the human hepatitis B virus (HBV) X-gene product was investigated. We demonstrate a new property for the HBV X-gene, the strong transcriptional trans-activation of promoters for class III genes. The stimulation of RNA polymerase III (pol III) as well as pol II promoters is shown in cells transiently transfected with the X-gene, and after its stable integration into hepatocytes. We demonstrate that X-gene containing cells stimulate the frequency of pol III transcription initiation by 20- to 40-fold, and accelerate the rate of formation of stable pol III initiation complexes in a manner indistinguishable from that of adenovirus E1a protein. Since the transcription factor TFIIIC has been shown to be limiting in the formation of stable pol III initiation complexes, template commitment experiments were performed which titrate the level of this factor in extracts. We show that X-protein containing extracts are far more efficient in forming stable pol III preinitiation complexes that cannot be competed away upon addition of a second template, indicating that TFIIIC is very probably a target of the X-protein. Thus, the HBV X-protein is apparently a member of a family of trans-activators capable of stimulating both pol II and III promoters, which includes the adenovirus E1a-protein and SV40 t antigen.