Identification of miR‐21 targets in breast cancer cells using a quantitative proteomic approach

MicroRNA (miRNA) play essential roles in biological processes ranging from cellular proliferation to apoptosis. Recently, miRNA have also been implicated in a number of diseases including cancers. However, the targets of most miRNA remain unknown. The majority of reports describing identification of miRNA targets are based on computational approaches or detection of altered mRNA levels despite the fact that most miRNA are thought to regulate their targets primarily at the level of translational inhibition in animals. miR‐21 is a miRNA with oncogenic activity that is involved in various cancer‐related processes such as invasion and migration. Given the importance of miR‐21 in tumorigenesis, we employed a quantitative proteomic strategy to systematically identify potential targets of miR‐21. By knocking down the expression of endogenous miR‐21 in MCF‐7 breast cancer cells, we observed an increase in the abundance of 58 proteins, implying that they could be potential targets of miR‐21. Validation of 12 of these candidate targets in luciferase assays showed that 6 of them were likely direct targets of miR‐21. Importantly, the mRNA of the majority of the candidate targets tested did not show a concomitant increase in abundance. Overall, our results demonstrate that miR‐21 affects the expression of many of its targets through translational inhibition and highlights the utility of proteomic approaches for identifying miRNA targets.     

Aromatic interactions in model peptide β-hairpins: ring current effects on proton chemical shifts

Crystal structures of eight peptide β-hairpins in the sequence Boc-Leu-Phe-Val-Xxx-Yyy-Leu-Phe-Val-OMe revealed that the Phe(2) and Phe(7) aromatic rings are in close spacial proximity, with the centroid-centroid distance (R(cen)) of 4.4-5.4 ? between the two phenyl rings. Proton NMR spectra in chloroform and methanol solution reveal a significant upfield shift of the Phe(7) C(δ,δ') H(2) protons (6.65-7.04 ppm). Specific assignments of the aromatic protons have been carried out in the peptide Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe (6). The anticipated ring current shifts have been estimated from the aromatic ring geometrics observed in crystals for all eight peptides. Only one of the C(δ,δ') H proton lies in the shielding zone with rapid ring flipping, resulting in averaging between the two extreme chemical shifts. An approximate estimate of the population of conformations, which resemble crystal state orientation, may be obtained. Key nuclear Overhauser effects (NOEs) between facing Phe side chains provide support for close similarity between the solid state and solution conformation. Temperature dependence of aromatic ring proton chemical shift and line widths for peptide 6 (Boc-Leu-Phe-Val-(D)Pro-(L)Pro-Leu-Phe-Val-OMe) and the control peptide Boc-Leu-Val-Val-(D)Pro-Gly-Leu-Phe-Val-OMe establish an enhanced barrier to ring flipping when the two Phe rings are in proximity. Modeling studies suggest that small, conformational adjustment about C(α)-C(β) (χ(1) ) and C(β)-C(γ) (χ(2) ) bonds of both the Phe residues may be required in order to permit unhindered, uncorrelated flipping of both the Phe rings. The maintenance of the specific aromatic ring orientation in organic solvents provides evidence for significant stabilizing interaction.     

Characterization of (Boc‐Cys/Sec‐NHMe)2 and (Boc‐Cys/Sec‐OMe)2: Evidence of local conformational difference between disulfide and diselenide

Conformations of disulfide and diselenide were compared in (Boc‐Cys/Sec‐NHMe)₂ and (Boc‐Cys/Sec‐OMe)₂ using X‐ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, density functional theory (DFT), and circular dichroism (CD) spectroscopy. Conformations of disulfide/diselenide in polypeptides are defined based on the sign of side chain torsion angle χ₃ (–CH₂–S/Se–S/Se–CH₂–); negative indicates left‐handed and positive indicates right‐handed orientation. In the crystals of (Boc‐Cys‐OMe)₂ and (Boc‐Sec‐OMe)₂, the disulfide exhibits a left‐handed and the diselenide a right‐handed orientation. Characterization of cystine and selenocystine derivatives in solution using ¹H‐NMR, natural abundant ⁷⁷Se NMR, 2D‐ROESY, and chemical shift analysis coupled to DMSO titration has indicated the symmetrical nature and antiparallel orientation of Cys/Sec residues about the disulfide/diselenide bridges. Structural calculations of cystine and selenocystine derivatives using DFT further support the antiparallel orientation of Cys/Sec residues about disulfide/diselenide. The far‐ultraviolet (UV) region CD spectra of cystine and selenocystine derivatives have exhibited the negative Cotton effect (CE) for disulfide and positive for diselenide confirming the difference in the conformational preference of disulfide and diselenide. In the previously reported polymorphic structure of (Boc‐Sec‐OMe)₂, the diselenide has right‐handed orientation. In the X‐ray structures of disulfide and diselenide analogues of Escherichia coli protein encoded by curli specific gene C (CgsC) retrieved from Protein Databank (PDB), disulfide has left‐handed and the diselenide right‐handed orientation. The current report provides the evidence for the local conformational difference between a disulfide and a diselenide group under unconstrained conditions, which may be useful for the rational replacement of disulfide by diselenide in polypeptide chains.     

Tyrosine-heme ligation in heme-peptide complex: design based on conserved motif of catalase.

On the basis of evolutionary conservation of sequence in catalases, we have designed a heme-binding peptide (Ac-RLKSYTDTQISR12-(GGGG)-CRIVHC22-NH2) for the 'redox activity modulation' of heme. Heme-binding studies showed a blue-shifted Soret (369 nm) in the presence of TFE and a red-shifted Soret (418 nm) in the absence of TFE. These blue- and red-shifted Sorets suggest ligation through tyrosinate and histidine, respectively. This is the first designed peptide ligating to heme through tyrosine. NMR studies have confirmed that tyrosine ligation to heme in this heme-peptide complex occurs only in the presence of TFE. We suggest that TFE induces helicity in the peptide and brings the arginine and tyrosine in proximity, resulting in ionization of the phenolic side chain of tyrosine. In the absence of TFE, the unstructured peptide lacks the intra-molecular Arg(+)Tyr(-) ion pair, allowing heme binding to histidine. This peptide has significant peroxidase activity though it does not have catalase activity.     

Comprehensive proteomic analysis of human bile

Bile serves diverse functions from metabolism to transport. In addition to acids and salts, bile is composed of proteins secreted or shed by the hepatobiliary system. Although there have been previous efforts to catalog biliary proteins, an in-depth analysis of the bile proteome has not yet been reported. We carried out fractionation of non-cancerous bile samples using a multipronged approach (SDS-PAGE, SCX and OFFGEL) followed by MS analysis on an LTQ-Orbitrap Velos mass spectrometer using high resolution at both MS and MS/MS levels. We identified 2552 proteins - the largest number of proteins reported in human bile till date. To our knowledge, there are no previous studies employing high-resolution MS reporting a more detailed catalog of any body fluid proteome in a single study. We propose that extensive fractionation coupled to high-resolution MS can be used as a standard methodology for in-depth characterization of any body fluid. This catalog should serve as a baseline for the future studies aimed at discovering biomarkers from bile in gallbladder, hepatic, and biliary cancers.     

A proteogenomic approach to map the proteome of an unsequenced pathogen - Leishmania donovani

Visceral leishmaniasis or kala azar is the most severe form of leishmaniasis and is caused by the protozoan parasite Leishmania donovani. There is no published report on L. donovani genome sequence available till date, although the genome sequences of three related Leishmania species are already available. Thus, we took a proteogenomic approach to identify proteins from two different life stages of L. donovani. From our analysis of the promastigote (insect) and amastigote (human) stages of L. donovani, we identified a total of 22,322 unique peptides from a homology-based search against proteins from three Leishmania species. These peptides were assigned to 3711 proteins in L. infantum, 3287 proteins in L. major, and 2433 proteins in L. braziliensis. Of the 3711 L. donovani proteins that were identified, the expression of 1387 proteins was detectable in both life stages of the parasite, while 901 and 1423 proteins were identified only in promastigotes and amastigotes life stages, respectively. In addition, we also identified 13 N-terminally and one C-terminally extended proteins based on the proteomic data search against the six-frame translated genome of the three related Leishmania species. Here, we report results from proteomic profiling of L. donovani, an organism with an unsequenced genome.     

Absolute systems biology--measuring dynamics of protein modifications

Mass spectrometry is already established as the method of choice for protein identification. Hitherto, a major limitation of mass spectrometry was the inability to quantitate protein levels. With the development of novel methods, such as the AQUA method by Gerber et al., this hurdle has finally been overcome, unleashing the power of mass spectrometry to study protein dynamics in cells.