Structure of a C-terminal AHNAK peptide in a 1:2:2 complex with S100A10 and an acetylated N-terminal peptide of annexin A2

Role of calcium-sensor proteins in cell membrane repair

Cell membrane repair is a critical process used to maintain cell integrity and survival from potentially lethal chemical, and mechanical membrane injury. Rapid increases in local calcium levels due to a membrane rupture have been widely accepted as a trigger for multiple membrane-resealing models that utilize exocytosis, endocytosis, patching, and shedding mechanisms. Calcium-sensor proteins, such as synaptotagmins (Syt), dysferlin, S100 proteins, and annexins, have all been identified to regulate, or participate in, multiple modes of membrane repair. Dysfunction of membrane repair from inefficiencies or genetic alterations in these proteins contributes to diseases such as muscular dystrophy (MD) and heart disease. The present review covers the role of some of the key calcium-sensor proteins and their involvement in membrane repair.

S100A10 and its binding partners in depression and antidepressant actions

S100A10 (p11) is an emerging player in the neurobiology of depression and antidepressant actions. p11 was initially thought to be a modulator of serotonin receptor (5-HTR) trafficking and serotonergic transmission, though newly identified binding partners of p11 and neurobiological studies of these proteins have shed light on multifunctional roles for p11 in the regulation of glutamatergic transmission, calcium signaling and nuclear events related to chromatin remodeling, histone modification, and gene transcription. This review article focuses on direct binding partners of p11 in the brain including 5-HTRs, mGluR5, annexin A2, Ahnak, Smarca3, and Supt6h, as well as their roles in neuronal function, particularly in the context of depressive-like behavior as well as behavioral effects of antidepressant drug treatments in mice. In addition, we discuss neurobiological insights from recently uncovered p11 pathways in multiple types of neurons and non-neuronal cells and cast major remaining questions for future studies.

Annexin A2 and Ahnak control cortical NuMA-dynein localization and mitotic spindle orientation

Proper mitotic spindle orientation depends on the correct anchorage of astral microtubules to the cortex. It relies on the remodeling of the cell cortex, a process not fully understood. Annexin A2 (Anx2) is a protein known to be involved in cortical domain remodeling. Here, we report that in early mitosis, Anx2 recruits the scaffold protein Ahnak at the cell cortex facing spindle poles, and the distribution of both proteins is controlled by cell adhesion. Depletion of either protein or impaired cortical Ahnak localization result in delayed anaphase onset and unstable spindle anchoring, which leads to altered spindle orientation. We find that Ahnak is present in a complex with dynein-dynactin. Furthermore, Ahnak and Anx2 are required for dynein and NuMA proper cortical localization and dynamics. We propose that the Ahnak/Anx2 complex influences the cortical organization of the astral microtubule anchoring complex, and thereby mitotic spindle positioning in human cells.

Studying the Structures of Relaxed and Fuzzy Interactions: The Diverse World of S100 Complexes

S100 proteins are small, dimeric, Ca²⁺-binding proteins of considerable interest due to their associations with cancer and rheumatic and neurodegenerative diseases. They control the functions of numerous proteins by forming protein–protein complexes with them. Several of these complexes were found to display “fuzzy” properties. Examining these highly flexible interactions, however, is a difficult task, especially from a structural biology point of view. Here, we summarize the available in vitro techniques that can be deployed to obtain structural information about these dynamic complexes. We also review the current state of knowledge about the structures of S100 complexes, focusing on their often-asymmetric nature.

Actin Cytoskeletal Dynamics in Single-Cell Wound Repair

The plasma membrane protects the eukaryotic cell from its surroundings and is essential for cell viability; thus, it is crucial that membrane disruptions are repaired quickly to prevent immediate dyshomeostasis and cell death. Accordingly, cells have developed efficient repair mechanisms to rapidly reseal ruptures and reestablish membrane integrity. The cortical actin cytoskeleton plays an instrumental role in both plasma membrane resealing and restructuring in response to damage. Actin directly aids membrane repair or indirectly assists auxiliary repair mechanisms. Studies investigating single-cell wound repair have often focused on the recruitment and activation of specialized repair machinery, despite the undeniable need for rapid and dynamic cortical actin modulation; thus, the role of the cortical actin cytoskeleton during wound repair has received limited attention. This review aims to provide a comprehensive overview of membrane repair mechanisms directly or indirectly involving cortical actin cytoskeletal remodeling.

Down-Regulation of AHNAK2 Inhibits Cell Proliferation, Migration and Invasion Through Inactivating the MAPK Pathway in Lung Adenocarcinoma

AHNAK nucleoprotein 2 (AHNAK2) has been emerged as a crucial protein for neuroblast differentiation and cell migration, thereby involving in the development of various cancers. However, the specific molecular mechanism of AHNAK2 in lung adenocarcinoma is inconclusive. By accessing to the Oncomine dataset and GEPIA website, a higher expression level of AHNAK2 was observed in lung adenocarcinoma tissue samples. Overall survival (OS) curve plotted by Kaplan-Meier method showed that up-regulation of AHNAK2 was related with poor prognosis of lung adenocarcinoma patients. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis and western blot were conducted to examine the expression level of genes in lung adenocarcinoma cells. Through functional in vitro experiments, cell proliferation, migration and invasion were all suppressed after AHNAK2 knockdown using Cell counting kit-8 (CCK-8) assay, wound-healing and transwell analysis. Reduction of AHNAK2 decreased the apoptosis rate using flow cytometry analysis. Moreover, the key markers of MAPK pathway, p-MEK, p-ERK and p-P90RSK were decreased due to the transfection of si-AHNAK2 in A549 cells. U0126, a MEK inhibitor, showed the similar effects on MAPK-related protein levels with si-AHNAK2. To sum up, AHNAK2 is significantly increased in lung adenocarcinoma and plays a carcinogenic role by activating the MAPK signaling pathway, providing a novel insight and raising possibility for lung adenocarcinoma treatment.

Structural characterization of a dimeric complex between the short cytoplasmic domain of CEACAM1 and the pseudo tetramer of S100A10-Annexin A2 using NMR and molecular dynamics

AIIt, a heterotetramer of S100A10 (P11) and Annexin A2, plays a key role in calcium dependent, membrane associations with a variety of proteins. We previously showed that AIIt interacts with the short cytoplasmic domain (12 amino acids) of CEACAM1 (CEACAM1-SF). Since the cytoplasmic domains of CEACAM1 help regulate the formation of cis- or trans-dimers at the cell membrane, we investigated the possible role of their association with AIIt in this process. Using NMR and molecular dynamics, we show that AIIt and its pseudoheterodimer interacts with two molecules of short cytoplasmic domain isoform peptides, and that interaction depends on the binding motif 454-Phe-Gly-Lys-Thr-457 where Phe-454 binds in a hydrophobic pocket of AIIt, the null mutation Phe454Ala reduces binding by 2.5 fold, and the pseudophosphorylation mutant Thr457Glu reduces binding by three fold. Since these two residues in CEACAM1-SF were also found to play a role in the binding of calmodulin and G-actin at the membrane, we hypothesize a sequential set of three interactions are responsible for regulation of cis- to trans-dimerization of CEACAM1. The hydrophobic binding pocket in AIIt corresponds to a previously identified binding pocket for a peptide found in SMARCA3 and AHNAK, suggesting a conserved functional motif in AIIt allowing multiple proteins to reversibly interact with integral membrane proteins in a calcium dependent manner.

Flexible Players within the Sheaths: The Intrinsically Disordered Proteins of Myelin in Health and Disease

Myelin ensheathes selected axonal segments within the nervous system, resulting primarily in nerve impulse acceleration, as well as mechanical and trophic support for neurons. In the central and peripheral nervous systems, various proteins that contribute to the formation and stability of myelin are present, which also harbor pathophysiological roles in myelin disease. Many myelin proteins have common attributes, including small size, hydrophobic segments, multifunctionality, longevity, and regions of intrinsic disorder. With recent advances in protein biophysical characterization and bioinformatics, it has become evident that intrinsically disordered proteins (IDPs) are abundant in myelin, and their flexible nature enables multifunctionality. Here, we review known myelin IDPs, their conservation, molecular characteristics and functions, and their disease relevance, along with open questions and speculations. We place emphasis on classifying the molecular details of IDPs in myelin, and we correlate these with their various functions, including susceptibility to post-translational modifications, function in protein–protein and protein–membrane interactions, as well as their role as extended entropic chains. We discuss how myelin pathology can relate to IDPs and which molecular factors are potentially involved.

Role of S100 proteins in health and disease

The S100 family of proteins contains 25 known members that share a high degree of sequence and structural similarity. However, only a limited number of family members have been characterized in depth, and the roles of other members are likely undervalued. Their importance should not be underestimated however, as S100 family members function to regulate a diverse array of cellular processes including proliferation, differentiation, inflammation, migration and/or invasion, apoptosis, Ca²⁺ homeostasis, and energy metabolism. Here we detail S100 target protein interactions that underpin the mechanistic basis to their function, and discuss potential intervention strategies targeting S100 proteins in both preclinical and clinical situations.

Annexins and plasma membrane repair

Plasma membrane wound repair is a cell-autonomous process that is triggered by Ca²⁺ entering through the site of injury and involves membrane resealing, i.e., re-establishment of a continuous plasma membrane, as well as remodeling of the cortical actin cytoskeleton. Among other things, the injury-induced Ca²⁺ elevation initiates the wound site recruitment of Ca²⁺-regulated proteins that function in the course of repair. Annexins are a class of such Ca²⁺-regulated proteins. They associate with acidic phospholipids of cellular membranes in their Ca²⁺ bound conformation with Ca²⁺ sensitivities ranging from the low to high micromolar range depending on the respective annexin protein. Annexins accumulate at sites of plasma membrane injury in a temporally controlled manner and are thought to function by controlling membrane rearrangements at the wound site, most likely in conjunction with other repair proteins such as dysferlin. Their role in membrane repair, which has been evidenced in several model systems, will be discussed in this chapter.

An In-Depth View of the Porcine Trabecular Meshwork Proteome

The house swine (Sus scrofa domestica Linnaeus 1758) is an important model organism regarding the study of neurodegenerative diseases, especially ocular neuropathies such as glaucoma. This is due to the high comparability of the porcine and human eye regarding anatomy and molecular features. In the pathogenesis of glaucoma, the trabecular meshwork (TM) forms a key ocular component in terms of intraocular pressure (IOP) elevation. Thereby, functional TM abnormalities are correlated with distinct proteomic alterations. However, a detailed analysis of the TM proteome has not been realized so far. Since the porcine eye has high potential as a model system to study ocular diseases such as glaucoma, the present study focuses on the in-depth analysis of the porcine TM proteome. By use of a bottom-up (BU) mass spectrometric (MS) platform utilizing electrospray ionization liquid chromatography tandem MS (LC-ESI-MS/MS) considering database-dependent and peptide de novo sequencing, more than 3000 TM proteins were documented with high confidence (FDR < 1%). A distinct number of proteins with neuronal association were revealed. To the best to our knowledge, many of these protein species have not been reported for TM tissue before such as reelin, centlein and high abundant neuroblast differentiation-associated protein AHNAK (AHNAK). Thereby, AHNAK might play a superordinate role in the TM regarding proposed tissue involvement in barrier function. Also, a high number of secretory proteins could be identified. The generated TM proteomic landscape underlines a multifunctional character of the TM beyond representing a simple drainage system. Finally, the protein catalogue of the porcine TM provides an in-depth view of the TM molecular landscape and will serve as an important reference map in terms of glaucoma research utilizing porcine animal models, porcine TM tissues and/or cultured TM cells.

An alternative N-terminal fold of the intestine-specific annexin A13a induces dimerization and regulates membrane-binding

Annexin proteins function as Ca²⁺-dependent regulators of membrane trafficking and repair that may also modulate membrane curvature. Here, using high-resolution confocal imaging, we report that the intestine-specific annexin A13 (ANX A13) localizes to the tips of intestinal microvilli and determined the crystal structure of the ANX A13a isoform to 2.6 Å resolution. The structure revealed that the N terminus exhibits an alternative fold that converts the first two helices and the associated helix-loop-helix motif into a continuous α-helix, as stabilized by a domain-swapped dimer. We also found that the dimer is present in solution and partially occludes the membrane-binding surfaces of annexin, suggesting that dimerization may function as a means for regulating membrane binding. Accordingly, as revealed by in vitro binding and cellular localization assays, ANX A13a variants that favor a monomeric state exhibited increased membrane association relative to variants that favor the dimeric form. Together, our findings support a mechanism for how the association of the ANX A13a isoform with the membrane is regulated.

S100A10 and Cancer Hallmarks: Structure, Functions, and its Emerging Role in Ovarian Cancer

S100A10, which is also known as p11, is located in the plasma membrane and forms a heterotetramer with annexin A2. The heterotetramer, comprising of two subunits of annexin A2 and S100A10, activates the plasminogen activation pathway, which is involved in cellular repair of normal tissues. Increased expression of annexin A2 and S100A10 in cancer cells leads to increased levels of plasmin—which promotes the degradation of the extracellular matrix—increased angiogenesis, and the invasion of the surrounding organs. Although many studies have investigated the functional role of annexin A2 in cancer cells, including ovarian cancer, S100A10 has been less studied. We recently demonstrated that high stromal annexin A2 and high cytoplasmic S100A10 expression is associated with a 3.4-fold increased risk of progression and 7.9-fold risk of death in ovarian cancer patients. Other studies have linked S100A10 with multidrug resistance in ovarian cancer; however, no functional studies to date have been performed in ovarian cancer cells. This article reviews the current understanding of S100A10 function in cancer with a particular focus on ovarian cancer.

S100 proteins as therapeutic targets

The human genome codes for 21 S100 protein family members, which exhibit cell- and tissue-specific expression patterns. Despite sharing a high degree of sequence and structural similarity, the S100 proteins bind a diverse range of protein targets and contribute to a broad array of intracellular and extracellular functions. Consequently, the S100 proteins regulate multiple cellular processes such as proliferation, migration and/or invasion, and differentiation, and play important roles in a variety of cancers, autoimmune diseases, and chronic inflammatory disorders. This review focuses on the development of S100 neutralizing antibodies and small molecule inhibitors and their potential therapeutic use in controlling disease progression and severity.

Calmodulin as a protein linker and a regulator of adaptor/scaffold proteins

Calmodulin (CaM) is a universal regulator for a huge number of proteins in all eukaryotic cells. Best known is its function as a calcium-dependent modulator of the activity of enzymes, such as protein kinases and phosphatases, as well as other signaling proteins including membrane receptors, channels and structural proteins. However, less well known is the fact that CaM can also function as a Ca²⁺-dependent adaptor protein, either by bridging between different domains of the same protein or by linking two identical or different target proteins together. These activities are possible due to the fact that CaM contains two independently-folded Ca²⁺ binding lobes that are able to interact differentially and to some degree separately with targets proteins. In addition, CaM can interact with and regulates several proteins that function exclusively as adaptors. This review provides an overview over our present knowledge concerning the structural and functional aspects of the role of CaM as an adaptor protein and as a regulator of known adaptor/scaffold proteins.

AHNAK as a Prognosis Factor Suppresses the Tumor Progression in Glioma

PURPOSE

AHNAK is originally identified as a giant protein based on the estimated size of approximately 700 kDa. The aim of this study is to identify the role of AHNAK in the pathogenesis of glioma.

METHODS

We tested AHNAK mRNA level in a panel of six human glioma cell lines, and in 30 cases of normal brain tissues and 73 cases of glioma tissue samples using a qRT-PCR method. Further, we analyzed the relationship of AHNAK expression with clinicopathological characteristics in glioma patients. Meanwhile, we analyzed the relationship of expression of AHNAK and survival of glioma patients in survival analyses. Then, in vitro, we analyzed the biological effects of AHNAK in glioma cell lines (U87 and U251) including proliferation assay, cell transwell assay, and apoptosis. And in vivo, we examined the effects of AHNAK on tumor growth using xenograft model of human glioma cells in nude mice. Then we examined the expression of Ki-67-positive cells in these tumors.

RESULTS

We found that the mRNA levels of AHNAK were down-regulated in 4 of 6 human glioma cell lines, especially in U87 and U251 cell lines. Meanwhile, in glioma patients, a negative correlation was found between the expression of AHNAK and the glioma histopathology. And a low expression of AHNAK was a significant and independent prognostic factor for poor survival of glioma patients. Through over expression of AHNAK in both of U87 and U251, we demonstrated that overexpression of AHNAK could inhibit glioma cell proliferation and invasion, induce apoptosis, and inhibit in vivo glioma tumor growth and ki-67 expression.

CONCLUSIONS

The AHNAK acts as a potential tumor suppressor. Our study provides a preclinical basis for developing AHNAK as a reliable clinical prognostic indicator for glioma patients, and a new biomarker for treatment response, and a potentially therapeutic target in glioma management options.

Protective Function of Ahnak1 in Vascular Healing after Wire Injury

AIM

Vascular remodeling following injury substantially accounts for restenosis and adverse clinical outcomes. In this study, we investigated the role of the giant scaffold protein Ahnak1 in vascular healing after endothelial denudation of the murine femoral artery.

METHODS

The spatiotemporal expression pattern of Ahnak1 and Ahnak2 was examined using specific antibodies and real-time quantitative PCR. Following wire-mediated endothelial injury of Ahnak1-deficient mice and wild-type (WT) littermates, the processes of vascular healing were analyzed.

RESULTS

Ahnak1 and Ahnak2 showed a mutually exclusive vascular expression pattern, with Ahnak1 being expressed in the endothelium and Ahnak2 in the medial cells in naïve WT arteries. After injury, a marked increase of Ahnak1- and Ahnak2-positive cells at the lesion site became evident. Both proteins showed a strong upregulation in neointimal cells 14 days after injury. Ahnak1-deficient mice showed delayed vascular healing and dramatically impaired re-endothelialization that resulted in prolonged adverse vascular remodeling, when compared to the WT littermates.

CONCLUSION

The large scaffold and adaptor proteins Ahnak1 and Ahnak2 exhibit differential expression patterns and functions in naïve and injured arteries. Ahnak1 plays a nonredundant protective role in vascular healing.

Plasma Membrane Repair: A Central Process for Maintaining Cellular Homeostasis

Plasma membrane repair is a conserved cellular response mediating active resealing of membrane disruptions to maintain homeostasis and prevent cell death and progression of multiple diseases. Cell membrane repair repurposes mechanisms from various cellular functions, including vesicle trafficking, exocytosis, and endocytosis, to mend the broken membrane. Recent studies increased our understanding of membrane repair by establishing the molecular machinery contributing to membrane resealing. Here, we review some of the key proteins linked to cell membrane repair.

AHNAK2 Participates in the Stress-Induced Nonclassical FGF1 Secretion Pathway

FGF1 is a nonclassically released growth factor that regulates carcinogenesis, angiogenesis, and inflammation. In vitro and in vivo, FGF1 export is stimulated by cell stress. Upon stress, FGF1 is transported to the plasma membrane where it localizes prior to transmembrane translocation. To determine which proteins participate in the submembrane localization of FGF1 and its export, we used immunoprecipitation mass spectrometry to identify novel proteins that associate with FGF1 during heat shock. The heat shock-dependent association of FGF1 with the large protein AHNAK2 was observed. Heat shock induced the translocation of FGF1 and AHNAK2 to the cytoskeletal fraction. In heat-shocked cells, FGF1 and the C-terminal fragment of AHNAK2 colocalized with F-actin in the vicinity of the cell membrane. Depletion of AHNAK2 resulted in a drastic decrease of stress-induced FGF1 export but did not affect spontaneous FGF2 export and FGF1 release induced by the inhibition of Notch signaling. Thus, AHNAK2 is an important element of the FGF1 nonclassical export pathway.

Comprehensive Proteomic and Metabolomic Signatures of Nontypeable Haemophilus influenzae-Induced Acute Otitis Media Reveal Bacterial Aerobic Respiration in an Immunosuppressed Environment

A thorough understanding of the molecular details of the interactions between bacteria and host are critical to ultimately prevent disease. Recent technological advances allow simultaneous analysis of host and bacterial protein and metabolic profiles from a single small tissue sample to provide insight into pathogenesis. We used the chinchilla model of human otitis media to determine, for the first time, the most expansive delineation of global changes in protein and metabolite profiles during an experimentally induced disease. After 48 h of infection with nontypeable Haemophilus influenzae, middle ear tissue lysates were analyzed by high-resolution quantitative two-dimensional liquid chromatography-tandem mass spectrometry. Dynamic changes in 105 chinchilla proteins and 66 metabolites define the early proteomic and metabolomic signature of otitis media. Our studies indicate that establishment of disease coincides with actin morphogenesis, suppression of inflammatory mediators, and bacterial aerobic respiration. We validated the observed increase in the actin-remodeling complex, Arp2/3, and experimentally showed a role for Arp2/3 in nontypeable Haemophilus influenzae invasion. Direct inhibition of actin branch morphology altered bacterial invasion into host epithelial cells, and is supportive of our efforts to use the information gathered to modify outcomes of disease. The twenty-eight nontypeable Haemophilus influenzae proteins identified participate in carbohydrate and amino acid metabolism, redox homeostasis, and include cell wall-associated metabolic proteins. Quantitative characterization of the molecular signatures of infection will redefine our understanding of host response driven developmental changes during pathogenesis. These data represent the first comprehensive study of host protein and metabolite profiles in vivo in response to infection and show the feasibility of extensive characterization of host protein profiles during disease. Identification of novel protein targets and metabolic biomarkers will advance development of therapeutic and diagnostic options for treatment of disease.

Annexin A2 complexes with S100 proteins: structure, function and pharmacological manipulation

Annexin A2 (AnxA2) was originally identified as a substrate of the pp60v-src oncoprotein in transformed chicken embryonic fibroblasts. It is an abundant protein that associates with biological membranes as well as the actin cytoskeleton, and has been implicated in intracellular vesicle fusion, the organization of membrane domains, lipid rafts and membrane-cytoskeleton contacts. In addition to an intracellular role, AnxA2 has been reported to participate in processes localized to the cell surface including extracellular protease regulation and cell-cell interactions. There are many reports showing that AnxA2 is differentially expressed between normal and malignant tissue and potentially involved in tumour progression. An important aspect of AnxA2 function relates to its interaction with small Ca²⁺-dependent adaptor proteins called S100 proteins, which is the topic of this review. The interaction between AnxA2 and S100A10 has been very well characterized historically; more recently, other S100 proteins have been shown to interact with AnxA2 as well. The biochemical evidence for the occurrence of these protein interactions will be discussed, as well as their function. Recent studies aiming to generate inhibitors of S100 protein interactions will be described and the potential of these inhibitors to further our understanding of AnxA2 S100 protein interactions will be discussed.

Structure of the S100A4/myosin-IIA complex

Background

S100A4, a member of the S100 family of Ca²⁺-binding proteins, modulates the motility of both non-transformed and cancer cells by regulating the localization and stability of cellular protrusions. Biochemical studies have demonstrated that S100A4 binds to the C-terminal end of the myosin-IIA heavy chain coiled-coil and disassembles myosin-IIA filaments; however, the mechanism by which S100A4 mediates myosin-IIA depolymerization is not well understood.

Results

We determined the X-ray crystal structure of the S100A4Δ8C/MIIA^1908-1923 peptide complex, which showed an asymmetric binding mode for the myosin-IIA peptide across the S100A4 dimer interface. This asymmetric binding mode was confirmed in NMR studies using a spin-labeled myosin-IIA peptide. In addition, our NMR data indicate that S100A4Δ8C binds the MIIA^1908-1923 peptide in an orientation very similar to that observed for wild-type S100A4. Studies of complex formation using a longer, dimeric myosin-IIA construct demonstrated that S100A4 binding dissociates the two myosin-IIA polypeptide chains to form a complex composed of one S100A4 dimer and a single myosin-IIA polypeptide chain. This interaction is mediated, in part, by the instability of the region of the myosin-IIA coiled-coil encompassing the S100A4 binding site.

Conclusion

The structure of the S100A4/MIIA^1908-1923 peptide complex has revealed the overall architecture of this assembly and the detailed atomic interactions that mediate S100A4 binding to the myosin-IIA heavy chain. These structural studies support the idea that residues 1908–1923 of the myosin-IIA heavy chain represent a core sequence for the S100A4/myosin-IIA complex. In addition, biophysical studies suggest that structural fluctuations within the myosin-IIA coiled-coil may facilitate S100A4 docking onto a single myosin-IIA polypeptide chain.

Novel interactions of the TRTK12 peptide with S100 protein family members: specificity and thermodynamic characterization

The S100 protein family consists of small, dimeric proteins that exert their biological functions in response to changing calcium concentrations. S100B is the best-studied member and has been shown to interact with more than 20 binding partners in a calcium-dependent manner. The TRTK12 peptide, derived from the consensus binding sequence for S100B, has previously been found to interact with S100A1 and has been proposed to be a general binding partner of the S100 family. To test this hypothesis and gain a better understanding of the specificity of binding for the S100 proteins, 16 members of the human S100 family were screened against this peptide and its alanine variants. Novel interactions were found with only two family members, S100P and S100A2, indicating that TRTK12 selectively interacts with a small subset of the S100 proteins. Substantial promiscuity was observed in the binding site of S100B thereby accommodating variations in the peptide sequence, while S100A1, S100A2, and S100P exhibited larger differences in the binding constants for the TRTK12 alanine variants. This suggests that single-point substitutions can be used to selectively modulate the affinity of TRTK12 peptides for individual S100 proteins. This study has important implications for the rational drug design of inhibitors for the S100 proteins, which are involved in a variety of cancers and neurodegenerative diseases.