The I-Ag7 MHC class II molecule linked to murine diabetes is a promiscuous peptide binder

Susceptibility to insulin-dependent diabetes mellitus is linked to MHC class II genes. The only MHC class II molecule expressed by nonobese diabetic (NOD) mice, I-Ag7, shares a common alpha-chain with I-Ad but has a peculiar beta-chain. As with most beta-chain alleles linked to diabetes susceptibility, I-Ag7 contains a nonaspartic residue at position beta57. We have produced large amounts of empty I-Ag7 molecules using a fly expression system to characterize its biochemical properties and peptide binding by phage-displayed peptide libraries. The identification of a specific binding peptide derived from glutamic acid decarboxylase (GAD65) has allowed us to crystallize and obtain the three-dimensional structure of I-Ag7. Structural information was critical in evaluating the binding studies. I-Ag7, like I-Ad, appears to be very promiscuous in terms of peptide binding. Their binding motifs are degenerate and contain small and/or small hydrophobic residues at P4 and P6 of the peptide, a motif frequently found in most globular proteins. The degree of promiscuity is increased for I-Ag7 over I-Ad as a consequence of a larger P9 pocket that can specifically accommodate negatively charged residues, as well as possibly residues with bulky side chains. So, although I-Ad and I-Ag7 are structurally closely related, stable molecules and good peptide binders, they differ functionally in their ability to bind significantly different peptide repertoires that are heavily influenced by the presence or the absence of a negatively charged residue at position 57 of the beta-chain. These characteristics link I-Ag7 with autoimmune diseases, such as insulin-dependent diabetes mellitus.     

Structural requirements for the interaction between peptide antigens and I-Ed molecules

We have analyzed the structural characteristics of the interaction between I-Ed molecules and their peptide ligands. It was found that unrelated good I-Ed binders share structurally similar "core" regions that were experimentally demonstrated to be crucial for binding to I-Ed molecules. Single amino acid substitution analogues of one good I-Ed binder, hen egg lysozyme 107-116, were analyzed for their capacity to bind to I-Ed molecules and to activate two different I-Ed-restricted T cell hybridomas. The results illustrate the great permissiveness of I-Ed-peptide interaction and the great specificity of T cell recognition. It was concluded from these analyses that basic residues on the peptide molecule play a crucial role in binding to I-Ed. This contrasts with the structural requirements for binding to the other Iad isotype, I-Ad, the crucial hydrophobic residues. Thus, different class II molecules of the same MHC haplotype may have rather distinct peptide binding specificities, thereby expanding the repertoire of possible immunogenic peptides presented for T cell recognition.     

Prediction of short linear protein binding regions

Short linear motifs in proteins (typically 3-12 residues in length) play key roles in protein-protein interactions by frequently binding specifically to peptide binding domains within interacting proteins. Their tendency to be found in disordered segments of proteins has meant that they have often been overlooked. Here we present SLiMPred (short linear motif predictor), the first general de novo method designed to computationally predict such regions in protein primary sequences independent of experimentally defined homologs and interactors. The method applies machine learning techniques to predict new motifs based on annotated instances from the Eukaryotic Linear Motif database, as well as structural, biophysical, and biochemical features derived from the protein primary sequence. We have integrated these data sources and benchmarked the predictive accuracy of the method, and found that it performs equivalently to a predictor of protein binding regions in disordered regions, in addition to having predictive power for other classes of motif sites such as polyproline II helix motifs and short linear motifs lying in ordered regions. It will be useful in predicting peptides involved in potential protein associations and will aid in the functional characterization of proteins, especially of proteins lacking experimental information on structures and interactions. We conclude that, despite the diversity of motif sequences and structures, SLiMPred is a valuable tool for prioritizing potential interaction motifs in proteins.     

Comparison of structural requirements for interaction of the same peptide with I-Ek and I-Ed molecules in the activation of MHC class II-restricted T cells.

We have analyzed the interaction of the hen egg-white lysozyme (HEL) peptide 107-116 with the MHC class II molecule I-Ek, using truncated and single residue substitution analogues to measure activation of I-Ek-restricted, 107-116-specific T cell hybridomas and competition for Ag presentation by I-Ek molecules. These results have been compared with previous findings on the interaction of the same peptide with the I-Ed molecule. Stimulation of T cell hybridomas by truncated peptides defines the sequence 108-116 as the minimum epitope necessary for activation of both I-Ek- and I-Ed-restricted T cell hybridomas. Substitution analysis pinpoints three residues (V109, A110, and K116) in the sequence 108-116 as being critical for binding to I-Ek molecules and demonstrates the involvement of most other residues in recognition by T cells. Results previously obtained for binding of HEL 107-116 to I-Ed molecules indicated that peptide residues R112, R114, and K116 were critical for interaction with I-Ed. Comparison of these results indicates a difference in the likely MHC contact residues between the HEL sequence 108-116 and I-Ed or I-Ek molecules, suggesting that the same HEL peptide assumes a different conformation in the binding site of these two MHC molecules. This in turn affects residues interacting with the specific T cell receptor. According to the hypothetical tridimensional structure predicted for class II molecules, the difference in MHC contact residues observed within the sequence 108-116 can be related to polymorphic amino acids in the binding site of I-Ek and I-Ed molecules. A search through published binding data for a common pattern in this and other I-Ek-binding peptides has permitted us to derive a possible motif for predicting peptide binding to I-Ek molecules. This putative motif was tested by determining binding to I-Ek of an unbiased panel of about 150 synthetic peptides. Binding data indeed demonstrate the presence of this motif in the majority of good binders to I-Ek molecules.     

CTL recognition of a bulged viral peptide involves biased TCR selection

MHC class I molecules generally present peptides of 8-10 aa long, forming an extended coil in the HLA cleft. Although longer peptides can also bind to class I molecules, they tend to bulge from the cleft and it is not known whether the TCR repertoire has sufficient plasticity to recognize these determinants during the antiviral CTL response. In this study, we show that unrelated individuals infected with EBV generate a significant CTL response directed toward an HLA-B*3501-restricted, 11-mer epitope from the BZLF1 Ag. The 11-mer determinant adopts a highly bulged conformation with seven of the peptide side chains being solvent-exposed and available for TCR interaction. Such a complex potentially creates a structural challenge for TCR corecognition of both HLA-B*3501 and the peptide Ag. Surprisingly, unrelated B*3501 donors recognizing the 11-mer use identical or closely related alphabeta TCR sequences that share particular CDR3 motifs. Within the small number of dominant CTL clonotypes observed, each has discrete fine specificity for the exposed side chain residues of the peptide. The data show that bulged viral peptides are indeed immunogenic but suggest that the highly constrained TCR repertoire reflects a limit to TCR diversity when responding to some unusual MHC peptide ligands.     

C-terminal recognition by 14-3-3 proteins for surface expression of membrane receptors

Diverse functions of 14-3-3 proteins are directly coupled to their ability to interact with targeted peptide substrates. RSX(pS/pT)XP and RXPhiX(pS/pT)XP are two canonical consensus binding motifs for 14-3-3 proteins representing the two common binding modes, modes I and II, between 14-3-3 and internal peptides. Using a genetic selection, we have screened a random peptide library and identified a group of C-terminal motifs, termed SWTY, capable of overriding an endoplasmic reticulum localization signal and redirecting membrane proteins to cell surface. Here we report that the C-terminal SWTY motif, although different from mode I and II consensus, binds tightly to 14-3-3 proteins with a dissociation constant (K(D)) of 0.17 microM, comparable with that of internal canonical binding peptides. We show that all residues but proline in -SWTX-COOH are compatible for the interaction and surface expression. Because SWTY-like sequences have been found in native proteins, these results support a broad significance of 14-3-3 interaction with protein C termini. The C-terminal binding consensus, mode III, represents an expansion of the repertoire of 14-3-3-targeted sequences.     

Biological heterogeneity of the peptide-binding motif of the 70-kDa heat shock protein by surface plasmon resonance analysis

70-kDa heat shock protein family is a molecular chaperone that binds to a variety of client proteins and peptides in the cytoplasm. Several studies have revealed binding motifs between 70-kDa heat shock protein family and cytoplasmic proteins by conventional techniques such as phage display library screening. However, little is known about the binding motif based on kinetic parameters determined by surface plasmon resonance analysis. We investigated the major inducible cytosolic 70-kDa heat shock protein (Hsp70)-binding motif with the human leukocyte antigen B*2702-derived peptide Bw4 (RENLRIALRY) by using a Biacore system based on surface plasmon resonance analysis. The K(D) value of Hsp70-Bw4 interaction was 1.8 x 10(-6) m. Analyses with truncated Bw4 variant peptides showed the binding motif of Hsp70 to be seven residues, LRIALRY. To further study the characteristics of this motif, 126 peptides derived from Bw4, each with single amino acid substitution, were synthesized and analyzed for Hsp70 binding affinity. Interestingly, the Hsp70 binding affinity was abrogated when the residues were substituted for by acidic (Asp and Glu) ones at any position. In contrast, if the substitute residue was aromatic (Trp, Tyr, and Phe) or an Arg residue at any position, Hsp70 binding affinity was maintained. Thus, this study presents a new binding motif between Hsp70 and peptides derived from the natural protein human leukocyte antigen B*2702 and may also elucidate some characteristics of the Hsp70 binding characteristic, enhancing our understanding of Hsp70-binding determinants that may influence diverse cellular and physiological processes.     

Dual structural requirements for multilineage hematopoietic-suppressive activity of chemokine-derived peptides

Many chemokines have direct suppressive activity in vitro and in vivo on primitive hematopoietic cells. However, few chemokine-derived peptides have shown a significant activity in inhibiting hematopoiesis. Interestingly, a peptide derived from the 34-58 sequence of the CXC chemokine platelet factor 4 (PF4) produced a 30-40% inhibition of proliferation of murine hematopoietic progenitors (CFU-MK, CFU-GM, and BFU-E) in vitro, at concentrations of 30-60-fold lower than PF4. The aim of the present work was to define the structural parameters and motifs involved in conferring biological activity to the peptide PF4(34-58). Both structural predictions and determinations revealed a new helical motif that was further localized between residues 38 and 46. This helix was necessary for binding of the peptide and for permitting the functional DLQ motif at position 54-56 to activate the putative receptor site. Peptides lacking either the helical or the DLQ motif were devoid of inhibitory activity on the hematopoietic progenitors in vitro. However, among inactive peptides, only those having the helical motif counteracted the inhibition induced by the active peptide PF4(34-58). This suggested that the helix might be required for peptide interactions with a putative receptor site, whereas the DLQ motif would be implicated in the activation of this receptor. These results identify for the first time the dual requirements for the design of chemokine-derived peptides with high suppressive activity on hematopoiesis, as well as for the design of molecules with antagonistic action.     

T cells that recognize peptide sequences of self MHC class II molecules exist in syngeneic mice.

T cell reactivity toward self MHC class II molecules has been recognized in syngeneic MLR in a number of studies, where the T cells are believed to recognize the combination of self/nonself peptide and self MHC molecule. We investigated the stimulation of T cell proliferation by synthetic peptides of sequences corresponding to the first polymorphic amino terminal domain of alpha- and beta-chains of self I-A molecules. Both unprimed and primed T cells responded to a number of peptides of alpha 1 and beta 1 domains of self I-Ad molecules. The response was dependent on the presentation of I-Ad peptides by syngeneic APC and was blocked by anti-class II MHC mAb. Upon further investigation it was observed that I-Ad peptides could inhibit the stimulation of Ag-specific MHC class II-restricted T cell hybridoma due to self presentation of peptides rather than to direct binding of free peptides to the TCR, further supporting their affinity/interaction with intact self MHC class II molecules. The peptide I-A beta d 62-78 showed high affinity toward intact self MHC II molecule as determined by the inhibition of Ag-specific T cell stimulation and yet was nonstimulatory for syngeneic T cells, therefore representing an MHC determinant that may have induced self tolerance. Thus we have shown that strong T cell proliferative responses can be generated in normal mice against the peptides derived from self MHC class II molecules and these cells are part of the normal T cell repertoire. Therefore complete tolerance toward potentially powerful immunodominant but cryptic determinants of self Ag may not be necessary to prevent autoimmune diseases.     

Structural characterization of the interactions between palladin and α-actinin

The interaction between α-actinin and palladin, two actin-cross-linking proteins, is essential for proper bidirectional targeting of these proteins. As a first step toward understanding the role of this complex in organizing cytoskeletal actin, we have characterized binding interactions between the EF-hand domain of α-actinin (Act-EF34) and peptides derived from palladin and generated an NMR-derived structural model for the Act-EF34/palladin peptide complex. The critical binding site residues are similar to an α-actinin binding motif previously suggested for the complex between Act-EF34 and titin Z-repeats. The structure-based model of the Act-EF34/palladin peptide complex expands our understanding of binding specificity between the scaffold protein α-actinin and various ligands, which appears to require an α-helical motif containing four hydrophobic residues, common to many α-actinin ligands. We also provide evidence that the Family X mutation in palladin, associated with a highly penetrant form of pancreatic cancer, does not interfere with α-actinin binding.     

Inhibition of protein interactions with the beta 2 sliding clamp of Escherichia coli DNA polymerase III by peptides from beta 2-binding proteins

The sliding clamp of the Escherichia coli replisome is now understood to interact with many proteins involved in DNA synthesis and repair. A universal interaction motif is proposed to be one mechanism by which those proteins bind the E. coli sliding clamp, a homodimer of the beta subunit, at a single site on the dimer. The numerous beta(2)-binding proteins have various versions of the consensus interaction motif, including a related hexameric sequence. To determine if the variants of the motif could contribute to the competition of the beta-binding proteins for the beta(2) site, synthetic peptides derived from the putative beta(2)-binding motifs were assessed for their abilities to inhibit protein-beta(2) interactions, to bind directly to beta(2), and to inhibit DNA synthesis in vitro. A hierarchy emerged, which was consistent with sequence similarity to the pentameric consensus motif, QL(S/D)LF, and peptides containing proposed hexameric motifs were shown to have activities comparable to those containing the consensus sequence. The hierarchy of peptide binding may be indicative of a competitive hierarchy for the binding of proteins to beta(2) in various stages or circumstances of DNA replication and repair.     

Prediction of binding sites of peptide recognition domains: an application on Grb2 and SAP SH2 domains

Determination of the binding motif and identification of interaction partners of the modular domains such as SH2 domains can enhance our understanding of the regulatory mechanism of protein-protein interactions. We propose here a new computational method to achieve this goal by integrating the orthogonal information obtained from binding free energy estimation and peptide sequence analysis. We performed a proof-of-concept study on the SH2 domains of SAP and Grb2 proteins. The method involves the following steps: (1) estimating the binding free energy of a set of randomly selected peptides along with a sample of known binders; (2) clustering all these peptides using sequence and energy characteristics; (3) extracting a sequence motif, which is represented by a hidden Markov model (HMM), from the cluster of peptides containing the sample of known binders; and (4) scanning the human proteome to identify binding sites of the domain. The binding motifs of the SAP and Grb2 SH2 domains derived by the method agree well with those determined through experimental studies. Using the derived binding motifs, we have predicted new possible interaction partners for the Grb2 and SAP SH2 domains as well as possible interaction sites for interaction partners already known. We also suggested novel roles for the proteins by reviewing their top interaction candidates.     

Large scale mass spectrometric profiling of peptides eluted from HLA molecules reveals N-terminal-extended peptide motifs

The majority of >2000 HLA class I molecules can be clustered according to overlapping peptide binding specificities or motifs recognized by CD8(+) T cells. HLA class I motifs are classified based on the specificity of residues located in the P2 and the C-terminal positions of the peptide. However, it has been suggested that other positions might be relevant for peptide binding to HLA class I molecules and therefore be used for further characterization of HLA class I motifs. In this study we performed large-scale sequencing of endogenous peptides eluted from K562 cells (HLA class I null) made to express a single HLA molecule from HLA-B*3501, -B*3502, -B*3503, -B*3504, -B*3506, or -B*3508. Using sequence data from >1,000 peptides, we characterized novel peptide motifs that include dominant anchor residues extending to all positions in the peptide. The length distribution of HLA-B35-bound peptides included peptides of up to 15 residues. Remarkably, we determined that some peptides longer than 11 residues represented N-terminal-extended peptides containing an appropriate HLA-B35 peptide motif. These results provide evidence for the occurrence of endogenous N-terminal-extended peptide-HLA class I configurations. In addition, these results expand the knowledge about the identity of anchor positions in HLA class I-associated peptides that can be used for characterization of HLA class I motifs.     

Novel mycobacteria antigen 85 complex binding motif on fibronectin

The members of the antigen 85 protein family (Ag85), consisting of members Ag85A, Ag85B, and Ag85C, are the predominantly secreted proteins of mycobacteria and possess the ability to specifically interact with fibronectin (Fn). Because Fn-binding proteins are likely to be important virulence factors of Mycobacterium spp., Ag85 may contribute to the adherence, invasion, and dissemination of organisms in host tissue. In this study, we reported the Fn binding affinity of Ag85A, Ag85B, and Ag85C from Mycobacterium avium subsp. paratuberculosis (MAP) (K(D) values were determined from 33.6 to 68.4 nm) and mapped the Ag85-binding motifs of Fn. Fn14, a type III module located on the heparin-binding domain II (Hep-2) of Fn, was discovered to interact with Ag85 from MAP. The peptide inhibition assay subsequently demonstrated that a peptide consisting of residues 17-26 from Fn14 ((17)SLLVSWQPPR(26), termed P17-26) could interfere with Ag85B binding to Fn (73.3% reduction). In addition, single alanine substitutions along the sequence of P17-26 revealed that the key residues involved in Ag85-Fn binding likely contribute through hydrophobic and charge interactions. Moreover, binding of Ag85 on Fn siRNA-transfected Caco2 cells was dramatically reduced (44.6%), implying the physiological significance of the Ag85-Fn interaction between mycobacteria and host cells during infection. Our results indicate that Ag85 binds to Fn at a novel motif and plays a critical role in mycobacteria adherence to host cells by initiating infection. Ag85 might serve as an important colonization factor potentially contributing to mycobacterial virulence.     

Identification of the WW domain-interaction sites in the unstructured N-terminal domain of EBV LMP 2A

Epstein-Barr virus latency is maintained by the latent membrane protein (LMP) 2A, which mimics the B-cell receptor (BCR) and perturbs BCR signaling. The cytoplasmic N-terminal domain of LMP2A is composed of 119 amino acids. The N-terminal domain of LMP2A (LMP2A NTD) contains two PY motifs (PPPPY) that interact with the WW domains of Nedd4 family ubiquitin-protein ligases. Based on our analysis of NMR data, we found that the LMP2A NTD adopts an overall random-coil structure in its native state. However, the region between residues 60 and 90 was relatively ordered, and seemed to form the hydrophobic core of the LMP2A NTD. This region resides between two PY motifs and is important for WW domain binding. Mapping of the residues involved in the interaction between the LMP2A NTD and WW domains was achieved by chemical shift perturbation, by the addition of WW2 and WW3 peptides. Interestingly, the binding of the WW domains mainly occurred in the hydrophobic core of the LMP2A NTD. In addition, we detected a difference in the binding modes of the two PY motifs against the two WW peptides. The binding of the WW3 peptide caused the resonances of five residues (Tyr(60), Glu(61), Asp(62), Trp(65), and Gly(66)) just behind the N-terminal PY motif of the LMP2A NTD to disappear. A similar result was obtained with WW2 binding. However, near the C-terminal PY motif, the chemical shift perturbation caused by WW2 binding was different from that due to WW3 binding, indicating that the residues near the PY motifs are involved in selective binding of WW domains. The present work represents the first structural study of the LMP2A NTD and provides fundamental structural information about its interaction with ubiquitin-protein ligase.     

Identification and characterization of peptides that interact with hepatitis B virus via the putative receptor binding site

A direct involvement of the PreS domain of the hepatitis B virus (HBV) large envelope protein, and in particular amino acid residues 21 to 47, in virus attachment to hepatocytes has been suggested by many previous studies. Several PreS-interacting proteins have been identified. However, they share few common sequence motifs, and a bona fide cellular receptor for HBV remains elusive. In this study, we aimed to identify PreS-interacting motifs and to search for novel HBV-interacting proteins and the long-sought receptor. PreS fusion proteins were used as baits to screen a phage display library of random peptides. A group of PreS-binding peptides were obtained. These peptides could bind to amino acids 21 to 47 of PreS1 and shared a linear motif (W1T2X3W4W5) sufficient for binding specifically to PreS and viral particles. Several human proteins with such a motif were identified through BLAST search. Analysis of their biochemical and structural properties suggested that lipoprotein lipase (LPL), a key enzyme in lipoprotein metabolism, might interact with PreS and HBV particles. The interaction of HBV with LPL was demonstrated by in vitro binding, virus capture, and cell attachment assays. These findings suggest that LPL may play a role in the initiation of HBV infection. Identification of peptides and protein ligands corresponding to LPL that bind to the HBV envelope will offer new therapeutic strategies against HBV infection.     

Design of N-substituted peptomer ligands for EVH1 domains

Ena/VASP proteins are implicated in cytoskeletal reorganization during actin-dependent motility processes. Recruitment to subcellular sites of actin polymerization is mediated by the highly conserved N-terminal EVH1 domain, which interacts with target proteins containing proline-rich motifs. The VASP EVH1 domain specifically binds peptides with the consensus motif FPPPP present in all its binding partners, including the Listerial ActA protein. Previous studies have shown that the Phe and first and final Pro residues are highly conserved and cannot be substituted with any other natural amino acid without significant loss of binding affinity. We have incorporated peptoid building blocks (sarcosine derived, non-natural amino acids) into the peptide SFEFPPPPTEDEL from the Listerial ActA protein and were able to substitute the most highly conserved residues of this motif while maintaining binding to the VASP EVH1 domain with affinities in the range of 45-180 microm. We then used NMR chemical shift perturbations to locate specific domain residues involved in particular interactions. These studies may open up the way for designing selective modulators of VASP function for biological studies and for the development of novel therapeutics for diseases involving pathologically altered cell adhesion or cell motility.     

Sequence-specific binding of prePhoD to soluble TatAd indicates protein-mediated targeting of the Tat export in Bacillus subtilis

The Tat (twin-arginine protein translocation) system initially discovered in the thylakoid membrane of chloroplasts has been described recently for a variety of eubacterial organisms. Although in Escherichia coli four Tat proteins with calculated membrane spanning domains have been demonstrated to mediate Tat-dependent transport, a specific transport system for twin-arginine signal peptide containing phosphodiesterase PhoD of Bacillus subtilis consists of one TatA/TatC (TatAd/TatCd) pair of proteins. Here, we show that TatAd was found beside its membrane-integrated localization in the cytosol were it interacted with prePhoD. prePhoD was efficiently co-immunoprecipitated by TatAd. Inefficient co-immunoprecipitation of mature PhoD and missing interaction to Sec-dependent and cytosolic peptides by TatAd demonstrated a particular role of the twin-arginine signal peptide for this interaction. Affinity of prePhoD to TatAd was interfered by peptides containing the twin-arginine motif but remained active when the arginine residues were substituted. The selective binding of TatAd to peptides derived from the signal peptide of PhoD elucidated the function of the twin-arginine motif as a target site for pre-protein TatAd interaction. Substitution of the binding motif demonstrated the pivotal role of basic amino acid residues for TatA binding. These features suggest that TatA interacts prior to membrane integration with its pre-protein substrate and could therefore assist targeting of twin-arginine pre-proteins.     

Processing and reactivity of T cell epitopes containing two cysteine residues from hen egg-white lysozyme (HEL74-90)

The Ag processing and structural requirements involved in the generation of a major T cell epitope from the hen egg-white lysozyme protein (HEL74-88), containing two cysteine residues at positions 76 and 80, were investigated. Several T cell hybridomas derived from both low responder (I-Ab) and high responder (I-Ak) mice recognize this region. These hybridomas are strongly responsive to native HEL, but unresponsive to the reduced and carboxymethylated protein. Air-oxidized HEL74-88 peptide was unable to bind I-Ak molecules and failed to stimulate T cells in the absence of intracellular Ag processing. Further functional competition assays showed that alkylation of cysteine residues with bulky methyl groups interferes with the contacts for the MHC class II molecules (I-Ak) of high responder mice and the I-Ab-restricted TCR of low responder mice. Serine substitutions of the cysteine residues of HEL74-88 either enhanced or abrogated T cell stimulation by the peptides without significant alterations in the class II binding. These results suggest that the cysteine residues of peptides must be free from disulfide bonding for efficient stimulation of T cells and yet frequently used modifications of cysteine residues may not be suitable for peptide-based vaccine development.     

The gamma-core motif correlates with antimicrobial activity in cysteine-containing kaliocin-1 originating from transferrins

Kaliocin-1 is a 31-residue peptide derived from human lactoferrin, and with antimicrobial properties that recapitulate those of its 611 amino acid parent holoprotein. As kaliocin-1 is a cysteine-stabilized peptide, it was of interest to determine whether it contained a multidimensional gamma-core signature recently identified as common to virtually all classes of disulfide-stabilized antimicrobial peptides. Importantly, sequence and structural analyses identified an iteration of this multidimensional antimicrobial signature in kaliocin-1. Further, the gamma-core motif was found to be highly conserved in the transferrin family of proteins across the phylogenetic spectrum. Previous studies suggested that the mechanism by which kaliocin-1 exerts anti-candidal efficacy depends on mitochondrial perturbation without cell membrane permeabilization. Interestingly, results of a yeast two-hybrid screening analysis identified an interaction between kaliocin-1 and mitochondrial initiation factor 2 in a Saccharomyces cerevisiae model system. Taken together, these data extend the repertoire of antimicrobial peptides that contain gamma-core motifs, and suggest that the motif is conserved within large native as well as antimicrobial peptide subcomponents of transferrin family proteins. Finally, these results substantiate the hypothesis that antimicrobial activity associated with host defense effector proteins containing a gamma-core motif may correspond to targets common to fungal mitochondria or their bacterial ancestors.