A Conserved Histidine Residue Drives Extein Dependence in an Enhanced Atypically Split Intein.

Split intein-mediated protein trans-splicing (PTS) is widely applied in chemical biology and biotechnology to carry out traceless and specific protein ligation. However, the external residues immediately flanking the intein (exteins) can reduce the splicing rate, thereby limiting certain applications of PTS. Splicing by a recently developed intein with atypical split architecture ("Cat") exhibits a stark dependence on the sequence of its N-terminal extein residues. Here, we further developed Cat using error-prone polymerase chain reaction (PCR) and a cell-based selection assay to produce Cat*, which exhibits greatly enhanced PTS activity in the presence of unfavorable N-extein residues. We then applied solution nuclear magnetic resonance spectroscopy and molecular dynamics simulations to explore how the dynamics of a conserved B-block histidine residue (His78) contribute to this extein dependence. The enhanced extein tolerance of Cat* reported here should expand the applicability of atypically split inteins, and the mechanism highlights common principles that contribute to extein dependence.

Protein-protein and protein-lipid interactions of pore-forming BCL-2 family proteins in apoptosis initiation.

Apoptosis is a common cell death program that is important in human health and disease. Signaling in apoptosis is largely driven through protein-protein interactions. The BCL-2 family proteins function in protein-protein interactions as key regulators of mitochondrial poration, the process that initiates apoptosis through the release of cytochrome c, which activates the apoptotic caspase cascade leading to cellular demolition. The BCL-2 pore-forming proteins BAK and BAX are the key executors of mitochondrial poration. We review the state of knowledge of protein-protein and protein-lipid interactions governing the apoptotic function of BAK and BAX, as determined through X-ray crystallography and NMR spectroscopy studies. BAK and BAX are dormant, globular α-helical proteins that participate in protein-protein interactions with other pro-death BCL-2 family proteins, transforming them into active, partially unfolded proteins that dimerize and associate with and permeabilize mitochondrial membranes. We compare the protein-protein interactions observed in high-resolution structures with those derived in silico by AlphaFold, making predictions based on combining experimental and in silico approaches to delineate the structural basis for novel protein-protein interaction complexes of BCL-2 family proteins.

A promiscuous split intein with expanded protein engineering applications.

The protein trans-splicing (PTS) activity of naturally split inteins has found widespread use in chemical biology and biotechnology. However, currently used naturally split inteins suffer from an "extein dependence," whereby residues surrounding the splice junction strongly affect splicing efficiency, limiting the general applicability of many PTS-based methods. To address this, we describe a mechanism-guided protein engineering approach that imbues ultrafast DnaE split inteins with minimal extein dependence. The resulting "promiscuous" inteins are shown to be superior reagents for protein cyclization and protein semisynthesis, with the latter illustrated through the modification of native cellular chromatin. The promiscuous inteins reported here thus improve the applicability of existing PTS methods and should enable future efforts to engineer promiscuity into other naturally split inteins.

An Atypical Mechanism of Split Intein Molecular Recognition and Folding.

Split inteins associate to trigger protein splicing in trans, a post-translational modification in which protein sequences fused to the intein pair are ligated together in a traceless manner. Recently, a family of naturally split inteins has been identified that is split at a noncanonical location in the primary sequence. These atypically split inteins show considerable promise in protein engineering applications; however, the mechanism by which they associate is unclear and must be different from that of previously characterized canonically split inteins due to unique topological restrictions. Here, we use a consensus design strategy to generate an atypical split intein pair (Cat) that has greatly improved activity and is amenable to detailed biochemical and biophysical analysis. Guided by the solution structure of Cat, we show that the association of the fragments involves a disorder-to-order structural transition driven by hydrophobic interactions. This molecular recognition mechanism satisfies the topological constraints of the intein fold and, importantly, ensures that premature chemistry does not occur prior to fragment complementation. Our data lead a common blueprint for split intein complementation in which localized structural rearrangements are used to drive folding and regulate protein-splicing activity.

Design of a Split Intein with Exceptional Protein Splicing Activity.

Protein trans-splicing (PTS) by split inteins has found widespread use in chemical biology and biotechnology. Herein, we describe the use of a consensus design approach to engineer a split intein with enhanced stability and activity that make it more robust than any known PTS system. Using batch mutagenesis, we first conduct a detailed analysis of the difference in splicing rates between the Npu (fast) and Ssp (slow) split inteins of the DnaE family and find that most impactful residues lie on the second shell of the protein, directly adjacent to the active site. These residues are then used to generate an alignment of 73 naturally occurring DnaE inteins that are predicted to be fast. The consensus sequence from this alignment (Cfa) demonstrates both rapid protein splicing and unprecedented thermal and chaotropic stability. Moreover, when fused to various proteins including antibody heavy chains, the N-terminal fragment of Cfa exhibits increased expression levels relative to other N-intein fusions. The durability and efficiency of Cfa should improve current intein based technologies and may provide a platform for the development of new protein chemistry techniques.

Small molecule SJ572946 activates BAK to initiate apoptosis.

Poration of the outer mitochondrial membrane by the effector BCL-2 proteins BAK and BAX initiates apoptosis. BH3-only initiators BID and BIM trigger conformational changes in BAK and BAX transforming them from globular dormant proteins to oligomers of the apoptotic pores. Small molecules that can directly activate effectors are being sought for applications in cancer treatment. Here, we describe the small molecule SJ572946, discovered in a fragment-based screen that binds to the activation groove of BAK and selectively triggers BAK activation over that of BAX in liposome and mitochondrial permeabilization assays. SJ572946 independently kills BAK-expressing BCL2allKO HCT116 cells revealing on target cellular activity. In combination with apoptotic inducers and BH3 mimetics, SJ572946 kills experimental cancer cell lines. SJ572946 also cooperates with the endogenous BAK activator BID in activating a misfolded BAK mutant substantially impaired in activation. SJ572946 is a proof-of-concept tool for probing BAK-mediated apoptosis in preclinical cancer research.

Solution NMR of polypeptides hyperpolarized by dynamic nuclear polarization.

Hyperpolarization of nuclear spins through techniques such as dynamic nuclear polarization (DNP) can greatly increase the signal-to-noise ratio in NMR measurements, thus eliminating the need for signal averaging. This enables the study of many dynamic processes which would otherwise not be amenable to study by NMR spectroscopy. A report of solid- to liquid-state DNP of a short peptide, bacitracin A, as well as of a full-length protein, L23, is presented here. The polypeptides are hyperpolarized at low temperature and dissolved for NMR signal acquisition in the liquid state in mixtures of organic solvent and water. Signal enhancements of 300-2000 are obtained in partially deuterated polypeptide when hyperpolarized on (13)C and of 30-180 when hyperpolarized on (1)H. A simulated spectrum is used to identify different resonances in the hyperpolarized (13)C spectra, and the relation between observed signal enhancement for various groups in the protein and relaxation parameters measured from the hyperpolarized samples is discussed. Thus far, solid- to liquid-state DNP has been used in conjunction with small molecules. The results presented here, however, demonstrate the feasibility of hyperpolarizing larger proteins, with potential applications toward the study of protein folding or macromolecular interactions.

Rapid determination of biosynthetic pathways using fractional isotope enrichment and high-resolution dynamic nuclear polarization enhanced NMR.

Carbon-13 NMR has traditionally been a method of choice for the determination of metabolic pathways. Through fractional labeling, (13)C spectra allow the identification of fragments incorporated as a unit into a biosynthesized molecule. The low sensitivity of (13)C spectroscopy is an impediment to such studies, especially if compounded with an often limited availability of biosynthesized molecules. Dynamic nuclear polarization (DNP) can increase the signal-to-noise ratio in magnetic resonance by several orders of magnitude, and in combination with high-resolution spectroscopy has the potential to increase the reach of this technique for metabolic profiling. Here, we present an application of high-resolution DNP enhanced NMR to the study of the biosynthetic pathways for membrane lipids. We show that fatty acid methyl esters are readily hyperpolarized in organic solvent. The resulting spectra resolve the various structural features of the chains, including atoms near the termini, as well as unsaturated and cyclopropyl groups. Peak patterns observed in fractionally labeled samples are explained by the way feed molecules are incorporated into fatty acid chains during synthesis. Differences in multiplet intensity between samples made from glucose and acetate feedstock mixtures further reveal metabolic preferences for these precursors in the biosynthesis of the product. In addition to the present study of lipid biosynthesis, high-resolution DNP-NMR of fractionally (13)C-labeled metabolites may present itself for the rapid determination of biosynthetic pathways in various biomedical applications, especially in cases of limited availability of the products of interest.

Polarization Transfer from Ligands Hyperpolarized by Dissolution Dynamic Nuclear Polarization for Screening in Drug Discovery.

Nuclear magnetic resonance (NMR) spectroscopy is a valuable technique for ligand screening, because it exhibits high specificity toward chemical structure and interactions. Dissolution dynamic nuclear polarization (DNP) is a recent advance in NMR methodology that enables the creation of non-equilibrium spin states, which can dramatically increase NMR sensitivity. Here, the transfer of such spin polarization from hyperpolarized ligand to protein is observed. Mixing hyperpolarized benzamidine with the serine protease trypsin, a "fingerprint" of enhanced protein signals is observed, which shows a different intensity profile than the equilibrium NMR spectrum of the protein, but coincides closely to the frequency profile of a saturation transfer difference (STD) NMR experiment. The DNP experiment benefits from hyperpolarization and enables observation of all frequencies in a single, rapid experiment. Based on these merits, it is an interesting alternative to the widely used STD experiment for identification of protein-ligand interactions.