| Name | Maria |
| Surname | Monti |
| Institution | University of Naples – Federico II |
| montimar@unina.it | |
| Address | UniNa: Department of Chemical Sciences, Via Cinthia, Complesso Monte Sant’Angelo 21, 80126 Naples, Italy. Ceinge: CEINGE Biotecnologie Avanzate, Via G. Salvatore 486, 80126 Naples, Italy |
| Resume | Download |
FCP1, a phosphatase specific of the carboxyl-terminal-domain of the large subunit of the RNA polymerase II (RNAPII), stimulates transcription elongation and it is required for general transcription and cell viability. To identify novel interacting proteins of FCP1, we used a human cell line expressing an epitope flagged FCP1 and proteins, which formed complexes with FCP1, were identified by mass spectrometry. We identified four proteins: RPB2 subunit of the RNAPII, the nuclear kinase, NDR1, the methyltransferase PRMT5 and the enhancer of rudimentary homologue (ERH) proteins. Intriguingly, both the PRMT5 and ERH proteins are interacting partners of the SPT5 elongation factor. Interactions of RPB2, ERH, NDR1 and PRMT5 with FCP1 were confirmed by co-immunoprecipitation or in vitro pull-down assays. Interaction between PRMT5 and FCP1 was further confirmed by co-immunoprecipitation of endogenous proteins. We found that FCP1 is a genuine substrate of PRMT5-methylation both in vivo and in vitro, and FCP1-associated PRMT5 can methylate histones H4 in vitro.
Formation of misfolded aggregates is an essential part of what proteins can do. The process of protein aggregation is central to many human diseases and any aggregating event needs to be prevented within a cell and in protein design. In order to aggregate, a protein needs to unfold its native state, at least partially. The conformational state that is prone to aggregate is difficult to study, due to its aggregating potential and heterogeneous nature. Here, we use a systematic approach of limited proteolysis, in combination with electrospray ionisation mass spectrometry, to investigate the regions that are most flexible and solvent-exposed within the native, ligand-bound and amyloidogenic states of muscle acylphosphatase (AcP), a protein previously shown to form amyloid fibrils in the presence of trifluoroethanol. Seven proteases with different degrees of specificity have been used for this purpose. Following exposure to the aggregating conditions, a number of sites along the sequence of AcP become susceptible to proteolytic digestion. The pattern of proteolytic cleavages obtained under these conditions is considerably different from that of the native and ligand-bound conformations and includes a portion within the N-terminal tail of the protein (residues 6-7), the region of the sequence 18-23 and the position 94 near the C terminus. There is a significant overlap between the regions of the sequence found to be solvent-exposed from the present study and those previously identified to be critical in the rate-determining steps of aggregation from protein engineering approaches. This indicates that a considerable degree of solvent exposure is a feature of the portions of a protein that initiate the process of aggregation.
RNA polymerase II transcription is associated with cyclic phosphorylation of the C-terminal domain (CTD) of the large subunit of RNA polymerase II. To date, FCP1 is the only specific CTD phosphatase, which is required for general transcription and cell viability. To identify FCP1-associated proteins, we constructed a human cell line expressing epitope-tagged FCP1. In addition to RAP74, a previously identified FCP1 interacting factor, we determined that FCP1-affinity purified extracts contain RNAPII that has either a hyper- or a hypo-phosphorylated CTD. In addition, by mass spectrometry of affinity purified FCP1-associated factors, we identified a novel FCP1-interacting protein, named MEP50, a recently described component of the methylosome complex that binds to the snRNP's Sm proteins. We found that FCP1 specifically interacts with components of the spliceosomal U small nuclear ribonucleoproteins. These results suggest a putative role of FCP1 CTD-phosphatase in linking the transcription elongation with the splicing process.
The interaction between the negative cis-element (AldA-NRE) and p97 repressor nuclear protein is a key step in modulating transcription of the human and mouse aldolase A (AldA) gene during the cell cycle and differentiation. In an attempt to clarify the role of transcriptional repression in regulating gene expression, we purified, from HeLa cells, the nuclear protein that specifically binds to the AldA negative regulatory element (NRE). Matrix-assisted laser desorption ionization-time of flight analysis and examination of protein profiles from the SwissProt database revealed that the previously defined p97 repressor is ZNF224, a zinc finger protein. We demonstrate that ZNF224, a Kruppel-like zinc finger transcription factor, is the repressor protein that specifically binds to the negative cis-element AldA-NRE and affects the AldA-NRE-mediated transcription.
Amyloid fibrils of patients treated with regular hemodialysis essentially consists of beta2-microglobulin (beta2-m) and its truncated species DeltaN6beta2-m lacking six residues at the amino terminus. The truncated fragment has a more flexible three-dimensional structure and constitutes an excellent candidate for the analysis of a protein in the amyloidogenic conformation. The surface topology of synthetic fibrils obtained from intact beta2-m and truncated DeltaN6beta2-m was investigated by the limited proteolysis/mass spectrometry approach that appeared particularly suited to gain insights into the structure of beta2-m within the fibrillar polymer. The distribution of prefential proteolytic sites observed in both fibrils revealed that the central region of the protein, which had been easily cleaved in the full-length globular beta2-m, was fully protected in the fibrillar form. In addition, the amino- and carboxy-terminal regions of beta2-m became exposed to the solvent in the fibrils, whereas they were masked completely in the native protein. These data indicate that beta2-m molecules in the fibrils consist of an unaccessible core comprising residues 20-87 with the strands I and VIII being not constrained in the fibrillar polymer and exposed to the proteases. Moreover, proteolytic cleavages observed in vitro at Lys 6 and Lys 19 reproduce specific cleavages that have to occur in vivo to generate the truncated forms of beta2-m occurring in natural fibrils. On the basis of these data, a possible mechanism for fibril formation from native beta2-m is discussed and an explanation for the occurrence of truncated protein species in natural fibrils is given.
Albumin Kenitra is a new type of genetic variant of human serum albumin that has been found in two members of a family of Sephardic Jews from Kenitra (Morocco). The slow-migrating variant and the normal protein were isolated by anion-exchange chromatography and, after treatment with CNBr, the digests were analyzed by two-dimensional electrophoresis in a polyacrylamide gel. The CNBr peptides of the variant were purified by reverse-phase high performance liquid chromatography and submitted to sequence analysis. Albumin Kenitra is peculiar because it has an elongated polypeptide chain, 601 residues instead of 585, and its sequence is modified beginning from residue 575. DNA structural studies showed that the variant is caused by a single-base insertion, an adenine at nucleotide position 15 970 in the genomic sequence, which leads to a frameshift with the subsequent translation to the first termination codon of exon 15. Mass spectrometric analyses revealed that the four additional cysteine residues of the variant form two new S-S bridges and showed that albumin Kenitra is partially O-glycosylated by a monosialylated HexHexNAc structure. This oligosaccharide chain has been located to Thr596 by amino-acid sequence analysis of the tryptic fragment 592-597.
The solution structure and stability of N-terminally truncated beta2-microglobulin (deltaN6beta2-m), the major modification in ex vivo fibrils, have been investigated by a variety of biophysical techniques. The results show that deltaN6beta2-m has a free energy of stabilization that is reduced by 2.5 kcal/mol compared to the intact protein. Hydrogen exchange of a mixture of the truncated and full-length proteins at microM concentrations at pH 6.5 monitored by electrospray mass spectrometry reveals that deltaN6beta2-m is significantly less protected than its wild-type counterpart. Analysis of deltaN6beta2-m by NMR shows that this loss of protection occurs in beta strands I, III, and part of II. At mM concentration gel filtration analysis shows that deltaN6beta2-m forms a series of oligomers, including trimers and tetramers, and NMR analysis indicates that strand V is involved in intermolecular interactions that stabilize this association. The truncated species of beta2-microglobulin was found to have a higher tendency to self-associate than the intact molecule, and unlike wild-type protein, is able to form amyloid fibrils at physiological pH. Limited proteolysis experiments and analysis by mass spectrometry support the conformational modifications identified by NMR and suggest that deltaN6beta2-m could be a key intermediate of a proteolytic pathway of beta2-microglobulin. Overall, the data suggest that removal of the six residues from the N-terminus of beta2-microglobulin has a major effect on the stability of the overall fold. Part of the tertiary structure is preserved substantially by the disulfide bridge between Cys25 and Cys80, but the pairing between beta-strands far removed from this constrain is greatly perturbed.
The topology of the thyroid transcription factor 1 homeodomain (TTF-1HD)-DNA complex was investigated by a strategy which combines limited proteolysis and selective chemical modification experiments with mass spectrometry methodologies. When limited proteolysis digestions were carried out with the protein in the absence or presence of its target oligonucleotide, differential peptide maps were obtained from which the amino acid residues involved in the interaction could be inferred. Similarly, selective acetylation of lysine residues in both the isolated and the complexed homeodomain allowed us to identify the amino acids protected by the interaction with DNA. Surface topology analysis of isolated TTF-1HD performed at neutral pH was in good agreement with the three-dimensional structure of the molecule as determined by NMR studies under acidic conditions. Minor differences were detected in the C-terminal region of the protein which, contrary to NMR data, showed no accessibility to proteases. Analysis of the complex provided an experimental validation of the model proposed on the basis of the homology with the homeodomain structures described so far. An increased accessibility of the C-terminal region was observed following the interaction, suggesting its displacement from the protein core by the oligonucleotide molecule. Comparative experiments with DNA fragments differing in sequence and binding capabilities highlighted structural differences among the complexes, mainly located in the N-terminal region of the homeodomain, thus accounting for their different dissociation constants.