Native mass spectrometry of proteoliposomes containing integral and peripheral membrane proteins.

Cellular membranes are critical to the function of membrane proteins, whether they are associated (peripheral) or embedded (integral) within the bilayer. While detergents have contributed to our understanding of membrane protein structure and function, there remains challenges in characterizing protein-lipid interactions within the context of an intact membrane. Here, we developed a method to prepare proteoliposomes for native mass spectrometry (MS) studies. We first use native MS to detect the encapsulation of soluble proteins within liposomes. We then find the peripheral Gß1γ2 complex associated with the membrane can be ejected and analyzed using native MS. Four different integral membrane proteins (AmtB, AqpZ, TRAAK, and TREK2), all of which have previously been characterized in detergent, eject from the proteoliposomes as intact complexes bound to lipids that have been shown to tightly associate in detergent, drawing a correlation between the two approaches. We also show the utility of more complex lipid environments, such as a brain polar lipid extract, and show TRAAK ejects from liposomes of this extract bound to lipids. These findings underscore the capability to eject protein complexes from membranes bound to both lipids and metal ions, and this approach will be instrumental in the identification of key protein-lipid interactions.

Polyamine detergents tailored for native mass spectrometry studies of membrane proteins.

Native mass spectrometry (MS) is a powerful technique for interrogating membrane protein complexes and their interactions with other molecules. A key aspect of the technique is the ability to preserve native-like structures and noncovalent interactions, which can be challenging depending on the choice of detergent. Different strategies have been employed to reduce charge on protein complexes to minimize activation and preserve non-covalent interactions. Here, we report the synthesis of a class of polyamine detergents tailored for native MS studies of membrane proteins. These detergents, a series of spermine covalently attached to various alkyl tails, are exceptional charge-reducing molecules, exhibiting a ten-fold enhanced potency over spermine. Addition of polyamine detergents to proteins solubilized in maltoside detergents results in improved, charge-reduced native mass spectra and reduced dissociation of subunits. Polyamine detergents open new opportunities to investigate membrane proteins in different detergent environments that have thwarted previous native MS studies.

The Effects of Charged Amino Acid Side-Chain Length on Diagonal Cross-Strand Interactions between Carboxylate- and Ammonium-Containing Residues in a ß-Hairpin.

The ß-sheet is one of the common protein secondary structures, and the aberrant aggregation of ß-sheets is implicated in various neurodegenerative diseases. Cross-strand interactions are an important determinant of ß-sheet stability. Accordingly, both diagonal and lateral cross-strand interactions have been studied. Surprisingly, diagonal cross-strand ion-pairing interactions have yet to be investigated. Herein, we present a systematic study on the effects of charged amino acid side-chain length on a diagonal ion-pairing interaction between carboxylate- and ammonium-containing residues in a ß-hairpin. To this end, 2D-NMR was used to investigate the conformation of the peptides. The fraction folded population and the folding free energy were derived from the chemical shift data. The fraction folded population for these peptides with potential diagonal ion pairs was mostly lower compared to the corresponding peptide with a potential lateral ion pair. The diagonal ion-pairing interaction energy was derived using double mutant cycle analysis. The Asp2-Dab9 (Asp: one methylene; Dab: two methylenes) interaction was the most stabilizing (-0.79 ± 0.14 kcal/mol), most likely representing an optimal balance between the entropic penalty to enable the ion-pairing interaction and the number of side-chain conformations that can accommodate the interaction. These results should be useful for designing ß-sheet containing molecular entities for various applications.

Longer charged amino acids favor ß-strand formation in hairpin peptides.

Interactions between charged amino acids significantly influence the structure and function of proteins. The encoded charged amino acids Asp, Glu, Arg, and Lys have different number of hydrophobic methylenes linking the backbone to the charged functionality. It remains to be fully understood how does this difference in the number of methylenes affect protein structure stability. Protein secondary structures are the fundamental three-dimensional building blocks of protein structures. ß-Sheet structures are particularly interesting, because these structures have been associated with a number of protein misfolding diseases. Herein, we report the effect of charged amino acid side chain length at two ß-strand positions individually on the stability of a ß-hairpin. The charged amino acids include side chains with a carboxylate, an ammonium, or a guanidinium group. The experimental peptides, fully folded reference peptides, and fully unfolded reference peptides were synthesized by solid phase peptide synthesis and analyzed by 2D NMR methods including TOCSY, DQF-COSY, and ROESY. Sequence specific assignments were performed for all peptides. The chemical shift data were used to derive the fraction folded population and the folding free energy for the experimental peptides. Results showed that the fraction folded population increased with increasing charged amino acid side chain length. These results should be useful for developing functional peptides that adopt the ß-conformation.

Swapping the Positions in a Cross-Strand Lateral Ion-Pairing Interaction between Ammonium- and Carboxylate-Containing Residues in a ß-Hairpin.

Cross-strand lateral ion-pairing interactions are important for antiparallel ß-sheet stability. Statistical studies suggested that swapping the position of cross-strand lateral residues should not significantly affect the interaction. Herein, we swapped the position of ammonium- and carboxylate-containing residues with different side-chain lengths in a cross-strand lateral ion-pairing interaction in a ß-hairpin. The peptides were analyzed by 2D-NMR. The fraction folded population and folding free energy were derived from the chemical shift data. The ion-pairing interaction energy was derived using double mutant cycle analysis. The general trends for the fraction folded population and interaction energetics remained similar upon swapping the position of the interacting charged residues. The most stabilizing cross-strand interactions were between short residues, similar to the unswapped study. However, the fraction folded populations for most of the swapped peptides were higher compared to the corresponding unswapped peptides. Furthermore, subtle differences in the ion-pairing interaction energy upon swapping were observed, most likely due to the "unleveled" relative positioning of the interacting residues created by the inherent right-handed twist of the structure. These results should be useful for developing functional peptides that rely on lateral ion-pairing interactions across antiparallel ß-strands.

Insertion of Pro-Hyp-Gly provides 2 kcal mol-1 stability but attenuates the specific assembly of ABC heterotrimeric collagen triple helices.

Collagen is a major structural component of the extracellular matrix and connective tissue. The key structural feature of collagen is the collagen triple helix, with a Xaa-Yaa-Gly (glycine) repeating pattern. The most frequently occurring triplet is Pro (proline)-Hyp (hydroxyproline)-Gly. The reversible thermal folding and unfolding of a series of heterotrimeric collagen triple helices with varying number of Pro-Hyp-Gly triplets were monitored by circular dichroism spectroscopy to determine the unfolding thermodynamic parameters Tm (midpoint transition temperature), ΔHTm (unfolding enthalpy), and ΔGunfold (unfolding free energy). The Tm and ΔGunfold of the heterotrimeric collagen triple helices increased with increasing number of Pro-Hyp-Gly triplets. The ΔGunfold increased by 2.0 ± 0.2 kcal mol-1 upon inserting one Pro-Hyp-Gly triplet into all three chains. The Tm difference between the most stable ABC combination and the second most stable BCC combination decreased with increasing number of Pro-Hyp-Gly triplets, even though the ΔGunfold difference remained the same. These results should be useful for tuning the stability of collagen triple helical peptides for hydrogel formation, recognition of denatured collagen triple helices as diagnostics and therapeutics, and targeted drug delivery.

Short-term anti-CD25 monoclonal antibody treatment and neogenetic CD4(+)CD25(high) regulatory T cells in kidney transplantation.

CD4(+)CD25(high) T cells named regulatory T (Treg) cells are generated and play a key role in the induction and maintenance of transplant tolerance in organ recipients. Interleukin-2 (IL-2) enhance the development of effector cells and is essential for generation of Treg cells. The effect of the anti-CD25 monoclonal antibody (anti-CD25mAb) induction therapy on the neogenetic CD4(+)CD25(high)Treg cells is important for therapeutic strategies in kidney transplant. To clarify the question, a prospective study was conducted in 21 living donor kidney transplant recipients who randomly divided into the anti-CD25mAb group (Daclizumab) with 11 patients and the control group with 10 patients. The frequency of CD4(+)CD25(high)Treg cells in total CD4(+) T cells was analyzed by flow cytometry and FoxP3 expression by RT-PCR in peripheral blood, and results were compared at day 0, 3, 13, 17, 27 posttransplantation. There was no significant difference in patient characteristics and allograft survival. The present study showed that in vivo antigen-specific Treg cells population were generated and expanded after transplant. Both groups showed a significant increase in the frequency of CD4(+)CD25(high)Treg cells and higher level of FoxP3 mRNA after transplantation while the serum creatinine declined. Compared with the control group, recipients with anti-CD25mAb injection had significantly lower percentage of CD4(+)CD25(high) in total CD4(+) cells (1.13%+/-0.13% vs 1.94%+/-0.22%, P=0.00; 3.75%+/-0.28% vs 7.11%+/-0.51%, P=0.00) on day 3, 17 after transplantation. While, the percentage was not significantly different on day 10, 27 (3.72%+/-0.19% vs 4.36%+/-0.28%, P=0.08; 7.84%+/-0.35% vs 8.56%+/-0.36%, P=0.16). However, there was not obvious difference in Foxp3 expression level associated with the source of the CD4(+)CD25(high)Treg cells at the different time point after transplant. Our data indicated that CD4(+)CD25(high)Treg cells were transiently affected by anti-CD25mAb, without depletion. In conclusion, the short-term treatment with anti-CD25mAb might not prevent the production, proliferation of neogenetic Treg cells in organ transplant.