Annexin A13 Research Articles

Genome-wide association study indicates novel associations of annexin A13 to secretory and GAS2L2 with mucous otitis media

To evaluate the genetics of chronic nonsuppurative otitis media (OM). We performed a genome-wide association study of 429,599 individuals included in the FinnGen study using three different case definitions: combined chronic nonsuppurative OM (7034 cases) (included serous and mucous chronic OM), mucous chronic OM (5953 cases), and secretory chronic OM (1689 cases). Individuals without otitis media were used as controls (417,745 controls). We used immunohistochemistry (IHC) of the murine middle ear to evaluate the expression of annexin A13. Four loci were significantly associated (p < 1.7 × 10−8) with nonsuppurative OM. Three out of the four association signals included missense variants in genes that may play a role in otitis media pathobiology. According to our subtype-specific analyses, one novel locus, located near ANXA13, was associated with secretory OM. Three loci (near TNFRSF13B, GAS2L2, and TBX1) were associated with mucous OM. Immunohistochemistry of murine middle ear samples revealed annexin A13 expression at the apical pole of the Eustachian tube epithelium as well as variable intensity of the secretory cells of the glandular structure in proximity to the Eustachian tube. We demonstrated that secretory and mucous OM have distinct and shared genetic associations. The association of GAS2L2 with ciliary epithelium function and the pathogenesis of dysfunctional mucosa in mucous OM is suggested. The abundant expression of annexin A13 in the Eustachian tube epithelium, along with its role in apical transport for the binding and transfer of phospholipids, indicates the role of annexin A13 and phospholipids in Eustachian tube dysfunction.

ANXA13 promotes cell proliferation and invasion and attenuates apoptosis in renal cell carcinoma

PurposeEmerging evidences have demonstrated that annexin A13 (ANXA13) is closely related to the occurrence and development of malignant tumors. However, the functions and underlying molecular mechanisms of ANXA13 in Clear cell renal cell carcinoma (ccRCC) have not been defined. Therefore, this study aimed to clarify the potential role of ANXA13 in regulating the proliferation, migration, invasion, cell cycle, and apoptosis of ccRCC cells. Patients and methodsThe quantitative real-time PCR (qRT-PCR) and western blotting was performed for detecting the ANXA13 expression in ccRCC tissues at the mRNA and protein levels, respectively. The GEPIA2 databases were used to derive data for analyzing the ANXA13 expression in pan-cancer and ccRCC clinical features. Cell Counting and colony formation assays, as well as flow cytometry, were used to detect cell proliferation, apoptosis, or cell cycle. The wound healing assay was used to evaluate the migration ability of cells, and the Trans-well assay was conducted to determine the cell invasiveness. ResultsANXA13 was upregulated in ccRCC cells and human ccRCC tissues. Furthermore, siANXA13 inhibited ccRCC cell proliferation, migration, invasion and induced cell apoptosis. ConclusionANXA13 was upregulated in ccRCC. ANXA13 promotes tumorigenic traits of ccRCC cell lines in vitro. ANXA13 is a potential novel biomarker and a potential therapeutic target in ccRCC.

RNA-binding is an ancient trait of the Annexin family.

Introduction: The regulation of intracellular functions in mammalian cells involves close coordination of cellular processes. During recent years it has become evident that the sorting, trafficking and distribution of transport vesicles and mRNA granules/complexes are closely coordinated to ensure effective simultaneous handling of all components required for a specific function, thereby minimizing the use of cellular energy. Identification of proteins acting at the crossroads of such coordinated transport events will ultimately provide mechanistic details of the processes. Annexins are multifunctional proteins involved in a variety of cellular processes associated with Ca2+-regulation and lipid binding, linked to the operation of both the endocytic and exocytic pathways. Furthermore, certain Annexins have been implicated in the regulation of mRNA transport and translation. Since Annexin A2 binds specific mRNAs via its core structure and is also present in mRNP complexes, we speculated whether direct association with RNA could be a common property of the mammalian Annexin family sharing a highly similar core structure. Methods and results: Therefore, we performed spot blot and UV-crosslinking experiments to assess the mRNA binding abilities of the different Annexins, using annexin A2 and c-myc 3'UTRs as well as c-myc 5'UTR as baits. We supplemented the data with immunoblot detection of selected Annexins in mRNP complexes derived from the neuroendocrine rat PC12 cells. Furthermore, biolayer interferometry was used to determine the KD of selected Annexin-RNA interactions, which indicated distinct affinities. Amongst these Annexins, Annexin A13 and the core structures of Annexin A7, Annexin A11 bind c-myc 3'UTR with KDs in the nanomolar range. Of the selected Annexins, only Annexin A2 binds the c-myc 5'UTR indicating some selectivity. Discussion: The oldest members of the mammalian Annexin family share the ability to associate with RNA, suggesting that RNA-binding is an ancient trait of this protein family. Thus, the combined RNA- and lipid-binding properties of the Annexins make them attractive candidates to participate in coordinated long-distance transport of membrane vesicles and mRNAs regulated by Ca2+. The present screening results can thus pave the way for studies of the multifunctional Annexins in a novel cellular context.

Annexin Expression in Cholangiocarcinoma, and Metastatic Pancreatic Ductal Adenocarcinoma “Is it be Helpful for Differential Diagnosis of These Tumors in the Liver?”

Background & Objective: Differential diagnosis between cholangiocarcinoma (CCA) and metastatic pancreatic ductal adenocarcinoma (PDA) in the liver is difficult and so far, no specific immunohistochemical marker is reported to differentiate these two tumors. Considering the existing literature, the level of expression of Annexins (Annexin A1, 10 and 13) have been studied for differential diagnosis between these two tumors by molecular methods and promising results have been reported. Therefore, in this study, we tried to investigate the immunohistochemical value of these three Annexins for the differential diagnosis of CCA and PDA in the liver.Methods: The articles that reported the research subject in 10 years (2009-2019), including 45 cases of CCA and 50 cases of metastatic PDA in the liver were evaluated considering the presence or absence of AnnexinA1 (ANXA1), Annexin A10 (ANXA10) and Annexin A13 (ANXA13) expression by immunohistochemistry, were investigated.Results & Conclusion: This study showed, ANXA1 was positive both in PDA and CCA, ANXA10 was positive in ~60% of PDA cases and ~40% of CCA cases, and ANXA13 was mostly negative in both groups. The best sensitivity was found in cytoplasmic and nuclear ANXA1 (80% and 84%, respectively) to distinguish PDA from CCA and vice versa. The best specificity was observed in ANXA10 and ANXA13 to distinguish PDA from CCA. Also, ANXA13 had the best specificity to distinguish CCA from PDA. Our investigations showed that, ANXA1 probably can classify positive cases correctly, but it cannot discriminate PDA from CCA. ANXA10 had fair sensitivity and specificity to discriminate PDA from CCA. ANXA13 apparently had a high specificity that can help to narrow-down the differential diagnoses.

Annexin Expression in Cholangiocarcinoma, and Metastatic Pancreatic Ductal Adenocarcinoma "Is it be Helpful for Differential Diagnosis of These Tumors in the Liver?"

Background & Objective:Differential diagnosis between cholangiocarcinoma (CCA) and metastatic pancreatic ductal adenocarcinoma (PDA) in the liver is difficult and so far, no specific immunohistochemical marker is reported to differentiate these two tumors. Considering the existing literature, the level of expression of Annexins (Annexin A1, 10 and 13) have been studied for differential diagnosis between these two tumors by molecular methods and promising results have been reported. Therefore, in this study, we tried to investigate the immunohistochemical value of these three Annexins for the differential diagnosis of CCA and PDA in the liver. Methods:The articles that reported the research subject in 10 years (2009-2019), including 45 cases of CCA and 50 cases of metastatic PDA in the liver were evaluated considering the presence or absence of AnnexinA1 (ANXA1), Annexin A10 (ANXA10) and Annexin A13 (ANXA13) expression by immunohistochemistry, were investigated.Results & Conclusion:This study showed, ANXA1 was positive both in PDA and CCA, ANXA10 was positive in ~60% of PDA cases and ~40% of CCA cases, and ANXA13 was mostly negative in both groups. The best sensitivity was found in cytoplasmic and nuclear ANXA1 (80% and 84%, respectively) to distinguish PDA from CCA and vice versa. The best specificity was observed in ANXA10 and ANXA13 to distinguish PDA from CCA. Also, ANXA13 had the best specificity to distinguish CCA from PDA. Our investigations showed that, ANXA1 probably can classify positive cases correctly, but it cannot discriminate PDA from CCA. ANXA10 had fair sensitivity and specificity to discriminate PDA from CCA. ANXA13 apparently had a high specificity that can help to narrow-down the differential diagnoses.

Annexin A13 predicts poor prognosis for lung adenocarcinoma patients and accelerates the proliferation and migration of lung adenocarcinoma cells by modulating epithelial-mesenchymal transition.

This study aimed to investigate the role of ANXA13 in lung adenocarcinoma (LUAD) growth, migration, and the underlying mechanisms. Firstly, in the TCGA dataset for LUAD, ANXA13 is found to be highly expressed in patients with LUAD and high expression of ANXA13 predicted poor outcomes in LUAD patients. Consistently, the data of qRT-PCR showed that the expression of ANXA13 was higher in LUAD cell lines (Calu-3, LTEP-a-2, and NCI-H1395) than that in normal lung cell line BEAS2B. Then, we performed gain- and loss of function of ANXA13 in NCI-H1395 and Calu-3 cells, respectively. The results displayed that deficiency of ANXA13 suppresses cell proliferation, invasion, and migration in Calu-3 cells and overexpression of ANXA13 augments cell proliferation, invasion, and migration in NCI-H1395 cells. Finally, it was found that silencing of ANXA13 obviously raised the protein expression levels of E-cadherin and reduced the protein levels of N-cadherin, Vimentin, and Snail in Calu-3 cells whereas overexpression of ANXA13 obviously receded the protein expression levels of E-cadherin and enhanced the protein levels of N-cadherin, Vimentin, and Snail in NCI-H1395 cells. This study analyzed the biological effects of ANXA13 in LUAD cells, indicating that ANXA13 could regard as a therapeutic target for LUAD.

Dectin-1 Binding to Annexins on Apoptotic Cells Induces Peripheral Immune Tolerance via NADPH Oxidase-2

Uptake of apoptotic cells (ACs) by dendritic cells (DCs) and induction of a tolerogenic DC phenotypeisan important mechanism for establishing peripheraltolerance to self-antigens. The receptors involved and underlying signaling pathways are not fully understood. Here, we identify Dectin-1 as a crucial tolerogenic receptor binding with nanomolar affinity to the core domain of several annexins (annexin A1, A5, and A13) exposed on ACs. Annexins bind to Dectin-1 on a site distinct from the interaction site of pathogen-derived β-glucans. Subsequent tolerogenic signaling induces selective phosphorylation of spleen tyrosine kinase (SYK), causing activation of NADPH oxidase-2 and moderate production of reactive oxygen species.Thus, mice deficient for Dectin-1 develop autoimmune pathologies (autoantibodies and splenomegaly) and generate stronger immune responses (cytotoxic Tcells) against ACs. Our data describe an important immunological checkpoint system and provide a link between immunosuppressive signals of ACs and maintenance of peripheral immune tolerance.

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.

Annexin A13 promotes tumor cell invasion in vitro and is associated with metastasis in human colorectal cancer.

PurposeAberrantly upregulated expression of selected members of annexin, a group of calcium- and membrane-binding proteins, have been found to be associated with metastasis, poor prognosis, and other clinical characteristics in colorectal cancer (CRC), the third most diagnosed cancer. However, ANXA13 (encoding protein annexin A13), the original founder gene of the annexin A family, has not been studied carefully as a potential prognostic biomarker in CRC.MethodsThe protein level of annexin A13 was determined by western blot in a panel of CRC cell lines. Tumor cell invasion was determined by a Matrigel in vitro invasion assay in selected CRC cells with either upregulated (via plasmid transfection) or downregulated (via siRNA treatment) expression of ANXA13. The clinicopathological features and prognostic values associated with ANXA13 expression were also evaluated in a group of 125 CRC patients.ResultsANXA13 was expressed at a high level in HCT116 and HT29 cells but undetected or at a lower level in SW620, SW48, and Rko cells. CRC cell invasion was promoted by ANXA13 overexpression in SW620 or Rko cells and was reduced by ANXA13 downregulation in HCT116 or HT29 cells. In CRC patients, ANXA13 expression levels correlated with lymph node metastasis and were associated with poor overall survival.ConclusionsANXA13 is associated with CRC cell invasion in vitro, and with lymph node metastasis and poor survival in CRC patients. Our results indicate that ANXA13 can be exploited as a biomarker for its diagnostic and prognostic values.

Abstract A115: Annexin A1: A novel gene target of miRNA-21

Abstract Introduction: Numerous studies have reported miRNA-21 (miR-21) to be deregulated in a wide variety of cancers, including breast. miR-21 regulates a number of processes and pathways relevant to tumorigenesis including PDCD4, a regulator of apoptosis; PTEN, a suppressor of PI3K/AKT signaling; Smad7 and MSH2, regulators of TGFβ signaling; and Sprouty 1, an inhibitor of ERK-MAPK signaling. Here we describe an additional gene target of miR-21, annexin A1 that may be particularly relevant in the biology of triple negative breast cancer. Methods: To identify post-transcriptionally regulated genes that may play a role in breast cancer progression, we screened proteome and transcriptome of MCF10A breast cancer progression panel, reported by Choong et al (2010) and Rhee et al (2008) respectively. This resulted in the identification of Annexin A1. To identify miRNAs that have the potential to regulate annexin A1, we performed an in silico screen for miRNAs targeting annexin A1 using the miRNA algorithm TargetScan, and found miR-21 as a potential regulator. To validate if miR-21 indeed regulates annexin A1 we performed transient transfections to modulate miRNA levels in cells and subsequently performed western blot analysis and luciferase assay to measure the annexin A1 protein and reporter levels respectively. To measure endogenous and modulated levels of miRNAs and their effect on mRNA targets we performed QPCR on RNA samples obtained cell lines. Results: Our in silico analysis, suggested that annexin A1 was a potential downstream target of miR-21 with probability of conserved site targeting (PCT ) &amp;lt; 0.1. To confirm that miR-21 directly targets annexin A1, we inserted their putative target sequence along with surrounding 3'UTR sequences, into a luciferase reporter vector. Co-transfection of the sequence-specific miR-21 inhibitor increased luciferase activity from the Annexin A1–3' UTR reporter vector by 1.8 fold to 2.5 fold relative to lipid only, in MCF7 cells that expresses relatively high levels of miR-21, whereas ~1.3 fold increase was caused by scramble sequence. In a complementary experiment, we transfected a sequence-specific miR-21 mimic (a double-stranded 21-nt form of miR-21 that is processed into a functional single-stranded miRNA in cells) along with Annexin A1-3'UTR reporter vector, in A293T cells, which expresses moderate levels of miR-21. The miR-21 mimic decreased Annexin A1-3'UTR driven luciferase activity by ~40% relative to lipid only, whereas ~15% increase was caused by scramble sequence. To validate that miR-21 regulates the endogenous expression of annexin A1 protein, we transfected sequence specific miR-21 inhibitors into MCF7 and 293T cells. The miR-21 inhibitor increased annexin A1 protein levels by ~50% in both cell lines compared to lipid only, whereas scramble sequences produced only a marginal increase (&amp;lt;10%) in annexin A1 protein. Our efforts to address if this regulation is posttranscriptional or transcriptional revealed that annexin A1 is posttranscriptionally regulated by miR-21 since we did not observe any significant increase in Annexin A1 (mRNA) levels by miR-21 inhibitors in MCF7 cells. Lastly, we examined the annexin A1 protein and miR-21 levels in a panel of breast cancer cell lines. These analyses revealed although there was no correlation in ER + cell lines, an inverse pattern between miR-21 and annexin A1 expression was suggested in triple negative breast cancer (TNBC) cell lines. Conclusions: Annexin A1 represents a novel target of miR-21. This interaction appears to be predominant in TNBC. These observations require validation in breast tissue samples to assess if this TNBC specific regulation of annexin A1 by miR-21 also holds true in patient samples. Citation Format: Anjana Bhardwaj, Nivetha Ganesan, Constance T. Albarracin, Isabelle Bedrosian. Annexin A1: A novel gene target of miRNA-21. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A115.

Novel Antiproliferative and Antioxidant Role of BjAnn1, a Mustard Annexin Protein in Human Glioblastoma Cell Lines

Glioblastoma represents the deadliest tumors of the central nervous systems and their bleak prognosis remains unchanged over the past few decades and requiring new treatments that curb the tumor progression. Cancer cells possess high reactive oxygen species (ROS) levels when compared to their normal control counterparts. We have earlier characterized the ROS-scavenging annexin protein, BjAnn1 from Brassica juncea and demonstrated its peroxidase activity. Since annexins are widely conserved across evolutionary lines with involvement in stress alleviation, we attempted to verify the effect of ectopic expression of ROS-scavenging BjAnn1 in human glioblastoma cell lines. The multiple sequence alignment revealed that BjAnn1 showed homology to human annexin A13 (73.18%), annexin A2 (70.66%), annexin A3 (72.53%) and annexin A8 (70.03%). Stable expression of BjAnn1 in U251 and U87 cells inhibited proliferation, migration and invading abilities in correlation with significant decrease in ROS levels in comparison to the empty vector (EV)-expressing controls. Multiple gene-profiling using cDNAPCR arrays revealed a prominent transcriptional upregulation of oxidative stress responsive genes including CCS, CYB, DUOX2, FOXM, GSS, MBL2, MT3, MTL5, NME5, PXDN, SOD2 and SOD3 in BjAnn1-expressing glioblastoma cells. Western blotting confirmed the significant increase in the expression of antioxidant enzymes SOD2 (MnSOD) and SOD3 (Cu/ZnSOD) in BjAnn1-expressing cells. We also observed a significant inhibition in nuclear translocation of p65 and p50, NF-κB reporter activity and expression of NF-κB-target genes in BjAnn1 expressing cells. In summary, our study shows a prominent tumor-suppressive role of ROS-scavenging BjAnn1 in glioblastoma cell lines suggesting it as a novel candidate for efficient gene therapy in glioblastoma.

Receptor-like Molecules on Human Intestinal Epithelial Cells Interact with an Adhesion Factor from Lactobacillus reuteri.

A surface protein of Lactobacillus reuteri, mucus adhesion-promoting protein (MapA), is considered to be an adhesion factor. MapA is expressed in L. reuteri strains and adheres to piglet gastric mucus, collagen type I, and human intestinal epithelial cells such as Caco-2. The aim of this study was to identify molecules that mediate the attachment of MapA from L. reuteri to the intestinal epithelial cell surface by investigating the adhesion of MapA to receptor-like molecules on Caco-2 cells. MapA-binding receptor-like molecules were detected in Caco-2 cell lysates by 2D-PAGE. Two proteins, annexin A13 (ANXA13) and paralemmin (PALM), were identified by MALDI TOF-MS. The results of a pull-down assay showed that MapA bound directly to ANXA13 and PALM. Fluorescence microscopy studies confirmed that MapA binding to ANXA13 and PALM was colocalized on the Caco-2 cell membrane. To evaluate whether ANXA13 and PALM are important for MapA adhesion, ANXA13 and PALM knockdown cell lines were established. The adhesion of MapA to the abovementioned cell lines was reduced compared with that to wild-type Caco-2 cells. These knockdown experiments established the importance of these receptor-like molecules in MapA adhesion.

Proteomic characterisation of Echinococcus granulosus hydatid cyst fluid from sheep, cattle and humans

The hydatid cyst fluid (HCF) of Echinococcus granulosus is a complex biological mixture containing a wide range of proteins of both parasite and host origin. Using a combination of in- and off-gel protein fractionation techniques and tandem mass spectrometry 130 HCF proteins were identified from fertile cysts of sheep and human origin and infertile cysts from cattle. Forty-eight proteins were of parasite origin including Antigen 5 and Antigen B--the most abundant parasite proteins, thioredoxin, low-density lipoprotein receptors, cyclophilin and ferritin. Across the three host species the identified HCF proteins were broadly similar although, based on spectral counts, three proteins, including an antigen B isoform, were more abundant in sheep HCF compared with the fluids of cattle and human origin. Eighty-two host proteins were identified in HCF from the three species. Host plasma proteins were the most abundant, although approximately thirty of the host proteins that were identified are not considered constituents of plasma. The identification of parasite heat shock proteins and annexin A13 exclusively in infertile cysts, along with an increased spectral count for cathepsin B, supports the hypothesis of increased cellular stress and apoptosis as the cause of their infertility.

Molecular cloning, functional characterization and localization of an annexin from a fish gill fluke Microcotyle sebastis (Platyhelminthes: Monogenea)

The full cDNA of an annexin gene from Microcotyle sebastis (MsANX) was cloned for the first time in monogeneans. The cDNA of MsANX comprises 1199bp with a 29bp 5′ untranslated region, an open reading frame of 1062bp, and a 108bp 3′ untranslated region. The recombinantly produced MsANX bound phosphatidylserine vesicles in the presence of Ca2+, whereas no MsANX was precipitated in the absence of free Ca2+. Phylogenetically, MsANX formed a cluster with human annexin A13, known as the earliest annexin in vertebrates and expressed mainly in the intestine. The localization of MsANX in M. sebastis was analyzed by Western blotting and immunohistochemistry using the antiserum raised against the recombinant MsANX. In Western blot analysis, rat antiserum bound to a protein corresponding to the MsANX in size when worm crude extracts were used as antigens, but no bands were detected by the antiserum when the excretory/secretory proteins of worms were used as antigens. In immunohistochemistry analysis, significant antibody binding annexin was found in the ovarian region, the pharynx and the intestinal caecum of the worm. Interestingly, the alimentary canal location of MsANX was similar to the location of human annexin A13, and further research is needed to trace evolutionary relationship among helminthic annexins and human annexin A13. Also it remains to be investigated whether immunization of naïve fish with the recombinant MsANX can induce protective immune responses against M. sebastis infection.

A study of proteins of annexin group in early fish development. IV. Identification of calcium-binding proteins in zebrafish egg by mass spectrometry

Calcium-binding proteins were isolated from zebrafish (Brachidanio rerio) eggs by the method of precipitation with acidic phospholipids in the presence of calcium ions. The revealed proteins separated by SDS-PAGE were identified by mass spectrometric tryptic peptide fingerprinting. The proteins included annexins A2a, A1a, A13.1, and A5. In addition, copine III, a member of the recently described copine family of C2 domain-containing proteins, was identified. Total RNA was analyzed in mature oocytes by RT-PCR and transcripts of two different annexin A13 forms (A13.1 and A13.2) as well as of annexin A3 were detected. Thus, the presence of both proteins and mRNAs of annexins has been shown in the zebrafish egg.

Structure–function relationship in annexin A13, the founder member of the vertebrate family of annexins

Annexin A13 is considered the original progenitor of the 11 other members of vertebrate annexins, a superfamily of calcium/phospholipid-binding proteins. It is highly tissue-specific, being expressed only in intestinal and kidney epithelial cells. Alternative splicing generates two isoforms, both of which bind to rafts. In view of the lack of structural information supporting the physiological role of this annexin subfamily, we have cloned, expressed and purified human annexin A13b to investigate its structural and functional properties. The N-terminus of annexin A13b: (i) destabilizes the conserved protein core, as deduced from the low melting temperature in the absence (44 degrees C) or presence of calcium (55 degrees C), and (ii) impairs calcium-dependent binding to acidic phospholipids, requiring calcium concentrations >400 microM. Truncation of the N-terminus restores thermal stability and decreases the calcium requirement for phospholipid binding, confirming its essential role in the structure-function relationship of this annexin. Non-myristoylated annexin A13b only binds to acidic phospholipids at high calcium concentrations. We show for the first time that myristoylation of annexin A13b enables the direct binding to phosphatidylcholine, raft-like liposomes and acidic phospholipids in a calcium-independent manner. The conformational switch induced by calcium binding, from a 'closed' to an 'open' conformation with exposure of Trp227, can be mimicked by a decrease in pH, a process that may be relevant for membrane interactions. Our studies confirm that the common structural and functional characteristics that are dependent on the protein core of vertebrate annexins are likely to be common conserved features, whereas their variable N-termini confer distinct functional properties on annexins, as we report for myristoylation of annexin A13b.