Periaxin and AHNAK nucleoprotein 2 form intertwined homodimers through domain swapping

AHNAKs roles in physiology and malignant tumors

The AHNAK family currently consists of two members, namely AHNAK and AHNAK2, both of which have a molecular weight exceeding 600 kDa. Homologous sequences account for approximately 90% of their composition, indicating a certain degree of similarity in terms of molecular structure and biological functions. AHNAK family members are involved in the regulation of various biological functions, such as calcium channel modulation and membrane repair. Furthermore, with advancements in biological and bioinformatics technologies, research on the relationship between the AHNAK family and tumors has rapidly increased in recent years, and its regulatory role in tumor progression has gradually been discovered. This article briefly describes the physiological functions of the AHNAK family, and reviews and analyzes the expression and molecular regulatory mechanisms of the AHNAK family in malignant tumors using Pubmed and TCGA databases. In summary, AHNAK participates in various physiological and pathological processes in the human body. In multiple types of cancers, abnormal expression of AHNAK and AHNAK2 is associated with prognosis, and they play a key regulatory role in tumor progression by activating signaling pathways such as ERK, MAPK, Wnt, and MEK, as well as promoting epithelial-mesenchymal transition.

AHNAK Contributes to Hepatocellular Carcinoma Growth by Interacting with IGF-1R

Neuroblast differentiation-associated protein AHNAK, a large structural scaffold protein, remains mysterious in biological processes. AHNAK plays a suppressive or progressive role in different types of cancers. To investigate the role of the AHNAK in hepatocellular carcinoma (HCC), cell viability assays were performed to determine the cell proliferation of the stable AHNAK-knockdown HepG2 cell line; co-immunoprecipitation (Co-IP) and liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) were performed on HCC and matched paracancerous (MPC) tissues. The Metascape platform was used for enrichment analyses; the "ComplexHeatmap" package was applied for cluster analyses and visualization. Co-IP, Western botting and immunofluorescence double staining were performed to assess the interactions between AHNAK and insulin-like growth factor 1 receptor (IGF-1R). AHNAK silencing reduced the viability of HepG2 cells; the interactome in HCC and MPC tissues enriched 204 pathways and processes, which partially reflected the signature of HCC field cancerization. AHNAK could co-localize and interact with IGF-1R. These results suggested that the AHNAK complex contributes to HCC growth, potentially by interacting with IGF-1R.

AHNAK2 is a biomarker and a potential therapeutic target of adenocarcinomas

Adenocarcinoma is the second largest histological type of cervical cancer, second only to cervical squamous cell carcinoma. At present, despite the clinical treatment strategies of cervical adenocarcinoma and cervical squamous cell carcinoma being similar, the outcome and prognosis of cervical adenocarcinoma are significantly poor. Therefore, it is urgent to find specific biomarker and therapeutic target for cervical adenocarcinoma. In this study, we aim to reveal and verify the potential biomarkers and therapeutic targets of cervical adenocarcinoma. Weighted correlation network analysis (WGCNA) reveals the differentially-expressed genes significantly related to the histological characteristics of the two cervical cancer subtypes. We select the genes with the top 20 significance for further investigation. Through microarray and immunohistochemical (IHC) analyses of a variety of tumor tissues, we find that among these 20 genes, AHNAK2 is highly expressed not only in cervical adenocarcinoma, but also in multiple of adenocarcinoma tissues, including esophagus, breast and colon, while not in normal gland tissues. In vitro, AHNAK2 knockdown significantly inhibits cell proliferation and migration of adenocarcinoma cell lines. In vivo, AHNAK2 knockdown significantly inhibits tumor progression and metastasis of various adenocarcinomas. RNA-sequencing and bioinformatics analyses suggest that the inhibitory effect of AHNAK2 knockdown on tumor progression is achieved by regulating DNA replication and upregulating Bim expression. Together, we demonstrate that AHNAK2 is a biomarker and a potential therapeutic target for adenocarcinomas.

Ahnak depletion accelerates liver regeneration by modulating the TGF-β/Smad signaling pathway

Ahnak, a large protein first identified as an inhibitor of TGF-β signaling in human neuroblastoma, was recently shown to promote TGF-β in some cancers. The TGF-β signaling pathway regulates cell growth, various biological functions, and cancer growth and metastasis. In this study, we used Ahnak knockout (KO) mice that underwent a 70% partial hepatectomy (PH) to investigate the function of Ahnak in TGF-β signaling during liver regeneration. At the indicated time points after PH, we analyzed the mRNA and protein expression of the TGF -β/Smad signaling pathway and cell cycle-related factors, evaluated the cell cycle through proliferating cell nuclear antigen (PCNA) immunostaining, analyzed the mitotic index by hematoxylin and eosin staining. We also measured the ratio of liver tissue weight to body weight. Activation of TGF-β signaling was confirmed by analyzing the levels of phospho-Smad 2 and 3 in the liver at the indicated time points after PH and was lower in Ahnak KO mice than in WT mice. The expression levels of cyclin B1, D1, and E1; proteins in the Rb/E2F transcriptional pathway, which regulates the cell cycle; and the numbers of PCNA-positive cells were increased in Ahnak KO mice and showed tendencies opposite that of TGF-β expression. During postoperative regeneration, the liver weight to body weight ratio tended to increase faster in Ahnak KO mice. However, 7 days after PH, both groups of mice showed similar rates of regeneration, following which their active regeneration stopped. Analysis of hepatocytes undergoing mitosis showed that there were more mitotic cells in Ahnak KO mice, consistent with the weight ratio. Our findings suggest that Ahnak enhances TGF-β signaling during postoperative liver regeneration, resulting in cell cycle disruption; this highlights a novel role of Ahnak in liver regeneration. These results provide new insight into liver regeneration and potential treatment targets for liver diseases that require surgical treatment. [BMB Reports 2022; 55(8): 401-406].

Exome sequencing in a child with neurodevelopmental disorder and epilepsy: Variant analysis of the AHNAK2 gene

Background

The AHNAK2 gene encodes a large nucleoprotein expressed in several tissues, including brain, squamous epithelia, smooth muscle, and neuropil. Its role in calcium signaling has been suggested and to date, clear evidence about its involvement in the pathogenesis of clinical disorders is still lacking.

Methods

Here, we report a female 24‐year‐old patient diagnosed with a cardio‐facio‐cutaneous‐like phenotype (CFC‐like), characterized by epilepsy, psychomotor development delay, atopic dermatitis, congenital heart disease, hypotonia, and facial dysmorphism, who is compound heterozygote for two missense mutations in the AHNAK2 gene detected by exome sequencing.

Results

This patient had no detectable variant in any of the genes known to be associated with the cardio‐facio‐cutaneous syndrome. Moreover, the mode of inheritance does not appear to be autosomal dominant, as it is in typical CFC syndrome. We have performed in silico assessment of mutation severity separately for each missense mutation, but this analysis excludes a severe effect on protein function. Protein structure predictions indicate the mutations are located in flexible regions possibly involved in molecular interactions.

Conclusion

We discuss an alternative interpretation on the potential involvement of the two missense mutations in the AHNAK2 gene on the expression of CFC‐like phenotype in this patient based on inter‐allelic complementation.

The Obscure Potential of AHNAK2

Simple Summary

AHNAK2 is a relatively newly discovered protein. It can interact with many other proteins. This protein is increased in cells of variety of different cancers. AHNAK2 may play a vital role in cancer formation. AHNAK2 may have a role in early detection of cancer. This obscure potential of AHNAK2 is being studied.

Abstract

AHNAK2 is a protein discovered in 2004, with a strong association with oncogenesis in various epithelial cancers. It has a large 616 kDa tripartite structure and is thought to take part in the formation of large multi-protein complexes. High expression is found in clear cell renal carcinoma, pancreatic ductal adenocarcinoma, uveal melanoma, and lung adenocarcinoma, with a relation to poor prognosis. Little work has been done in exploring the function and relation AHNAK2 has with cancer, with early studies showing promising potential as a future biomarker and therapeutic target.

Crystal and solution structures reveal oligomerization of individual capsid homology domains of Drosophila Arc

Synaptic plasticity is vital for brain function and memory formation. One of the key proteins in long-term synaptic plasticity and memory is the activity-regulated cytoskeleton-associated protein (Arc). Mammalian Arc forms virus-like capsid structures in a process requiring the N-terminal domain and contains two C-terminal lobes that are structural homologues to retroviral capsids. Drosophila has two isoforms of Arc, dArc1 and dArc2, with low sequence similarity to mammalian Arc, but lacking a large N-terminal domain. Both dArc isoforms are related to the Ty3/gypsy retrotransposon capsid, consisting of N- and C-terminal lobes. Structures of dArc1, as well as capsids formed by both dArc isoforms, have been recently determined. We carried out structural characterization of the four individual dArc lobe domains. As opposed to the corresponding mammalian Arc lobe domains, which are monomeric, the dArc lobes were all oligomeric in solution, indicating a strong propensity for homophilic interactions. A truncated N-lobe from dArc2 formed a domain-swapped dimer in the crystal structure, resulting in a novel dimer interaction that could be relevant for capsid assembly or other dArc functions. This domain-swapped structure resembles the dimeric protein C of flavivirus capsids, as well as the structure of histones dimers, domain-swapped transcription factors, and membrane-interacting BAK domains. The strong oligomerization properties of the isolated dArc lobe domains explain the ability of dArc to form capsids in the absence of any large N-terminal domain, in contrast to the mammalian protein.

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.

Bioinformatics analysis of the clinical value and potential mechanisms of AHNAK2 in papillary thyroid carcinoma

BACKGROUND

AHNAK2 has been recently reported as a biomarker in many cancers. However, a systematic investigation of AHNAK2 in papillary thyroid carcinoma (PTC) has not been conducted.

RESULTS

AHNAK2 is overexpressed in PTC tissues and could be an independent prognostic factor. AHNAK2 expression was significantly high in patients with advanced stage, advanced T classification, lymph node metastasis, increased BRAF mutations and decreased RAS mutations. Cell adhesion-, cell junction-, and immune-related pathways were the most frequently noted in gene set enrichment analysis. AHNAK2 expression in PTC was positively correlated with immune infiltration and negatively correlated with AHNAK2 methylation. AHNAK2 expression was significantly positively correlated with tumor progression and poor overall survival (OS) in pan-cancer patients.

CONCLUSIONS

AHNAK2 is a good biomarker for the diagnosis and prognosis of PTC. AHNAK2 may promote thyroid cancer progression through cell adhesion-, cell junction-, and immune-related pathways. Methylation may act as an upstream regulator to inhibit the expression and biological function of AHNAK2. Additionally, AHNAK2 has broad prognostic value in pan-cancer.

METHODS

Based on The Cancer Genome Atlas (TCGA) data, we screened AHNAK2-related genes through weighted gene coexpression network analysis and explored the clinical value and the potential mechanism of AHNAK2 in PTC by multiomics analysis.

AHNAK2 Is Associated with Poor Prognosis and Cell Migration in Lung Adenocarcinoma

BACKGROUND

Lung adenocarcinoma (LUAD), as the main subtype of lung cancer, is one of the common causes of cancer-related deaths worldwide. The AHNAK family is correlated with cell structure and migration, cardiac calcium channel signaling, and tumor metastasis. Previous studies showed AHNAK2 could promote tumor progression and cell migration in melanoma and renal clear cell carcinoma. However, the role of AHNAK2 in LUAD remains unknown.

METHODS

We examined the levels of AHNAK2 in pathological specimens and the database of Clinical Proteomic Tumor Analysis Consortium-Lung adenocarcinoma (CPTAC-LUAD), The Cancer Genome Atlas-Lung Adenocarcinoma (TCGA-LUAD), Gene Expression Omnibus dataset (GSE72094, GSE26939), and The Genotype-Tissue Expression (GTEx) of lung tissue samples. Univariate Cox regression, multivariate Cox regression, and Kaplan-Meier survival analysis were performed to reveal the relationship between AHNAK2 and prognosis. A nomogram was constructed to predict 2- or 3-year overall survival and validated via calibration curves, receiver operating characteristic (ROC) analysis, and decision curve analysis (DCA). Furthermore, Gene Ontology (GO) analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis were used to explore the functional role of AHNAK2 in lung adenocarcinoma. Finally, by transfecting siRNA, we examined the regulatory effect of AHNAK2 on cell migration.

RESULTS

The expression of AHNAK2 was upregulated in tumor samples and correlated with poor prognosis in LUAD patients. Nomogram with AHNAK2 and clinical parameters showed a good prediction in overall survival (OS), especially the 2-year OS. In addition, functional analyses and wound healing assay suggested that AHNAK2 might be involved in the regulation of migration in LUAD.

CONCLUSION

In summary, our study showed that AHNAK2 might be a novel biomarker in LUAD and revealed the potential mechanism of AHNAK2 in LUAD progression which could provide new insights for target therapy.

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.

Ezrin interacts with L-periaxin by the "head to head and tail to tail" mode and influences the location of L-periaxin in Schwann cell RSC96

In the peripheral nervous system (PNS), Schwann cells (SCs) are required for the myelination of axons. Periaxin (PRX), one of the myelination proteins expressed in SCs, is critical for the normal development and maintenance of PNS. As a member of the ERM (ezrin-radxin-moesin) protein family, ezrin holds our attention since their link to the formation of the nodes of Ranvier. Furthermore, PRX and ezrin are co-expressed in cytoskeletal complexes with periplakin and desmoyokin in lens fiber cells. In the present study, we observed that L-periaxin and ezrin interacted in a "head to head and tail to tail" mode in SC RSC96 through NLS3 region of L-periaxin with F3 subdomain of ezrin interaction, and the region of L-periaxin (residues 1368-1461) with ezrin (residues 475-557) interaction. A phosphorylation-mimicking mutation of ezrin resulted in L-periaxin accumulation on SC RSC96 membrane. Ezrin could inhibit the self-association of L-periaxin, and ezrin overexpression in sciatic nerve injury rats could facilitate the repair of impaired myelin sheath. Therefore, the interaction between L-periaxin and ezrin may adopt a close form to complete protein accumulation and to participate in myelin sheath maintenance.

AHNAK Nucleoprotein 2 Performs a Promoting Role in the Proliferation and Migration of Uveal Melanoma Cells

AHNAK nucleoprotein 2 (AHNAK2) is supposed to participate in calcium signaling and cytoarchitecture by directly interacting with some proteins. Recently, it was identified as a novel candidate oncogene in human tumors. The author's present study aimed to investigate the expression and biological function of AHNAK2 in uveal melanoma (UM). Based on microarray data of 63 UM patients that were downloaded from Gene Expression Omnibus database, the authors found that AHNAK2 expression is higher in UM primary tumor tissues from patients who developed metastases after enucleation than that in UM primary tumor tissues from patients without metastasis after enucleation. On the basis of the data obtained from The Cancer Genome Atlas database, they found that high AHNAK2 expression is closely associated with shorter overall survival time in UM patients. From quantitative reverse transcription polymerase chain reaction analyses, they revealed that the mRNA expression level of AHNAK2 was significantly upregulated in M17 and SP6.5 cell lines compared with that in D78. Functionally, knockdown of AHNAK2 using small interfering RNA in M17 and SP6.5 cells dramatically suppressed cell proliferation, migratory and invasive abilities, as well as inhibited the activation of phosphatidylinositol 3-kinase (PI3K) signaling pathway. Taken together, their results illustrated that AHNAK2 was upregulated in UM and plays a promoting role in the proliferation and migration of UM cells possibly via regulating PI3K signaling pathway.

Direct Binding of the Flexible C-Terminal Segment of Periaxin to β4 Integrin Suggests a Molecular Basis for CMT4F

The process of myelination in the nervous system requires a coordinated formation of both transient and stable supramolecular complexes. Myelin-specific proteins play key roles in these assemblies, which may link membranes to each other or connect the myelinating cell cytoskeleton to the extracellular matrix. The myelin protein periaxin is known to play an important role in linking the Schwann cell cytoskeleton to the basal lamina through membrane receptors, such as the dystroglycan complex. Mutations that truncate periaxin from the C terminus cause demyelinating peripheral neuropathy, Charcot-Marie-Tooth (CMT) disease type 4F, indicating a function for the periaxin C-terminal region in myelination. We identified the cytoplasmic domain of β4 integrin as a specific high-affinity binding partner for periaxin. The C-terminal region of periaxin remains unfolded and flexible when bound to the third fibronectin type III domain of β4 integrin. Our data suggest that periaxin is able to link the Schwann cell cytoplasm to the basal lamina through a two-pronged interaction via different membrane protein complexes, which bind close to the N and C terminus of this elongated, flexible molecule.

AHNAK suppresses tumour proliferation and invasion by targeting multiple pathways in triple-negative breast cancer

Background

AHNAK, also known as desmoyokin, is a giant protein with the molecular size of approximately 700 kDa and exerts diverse functions in different types of cancer.

Results

In the present study, we demonstrated that AHNAK mRNA levels were down-regulated in 7 out of 8 human breast cancer cell lines, especially in triple - negative breast cancer (TNBC) cell lines. Moreover, in patients with TNBC, the expression of AHNAK gene was inversely correlated with the tumor status (P = 0.015), lymph node status (P < 0.001), lymph node (LN) infiltration (P < 0.001) and TNM stage (P < 0.001). Moreover, down-regulated AHNAK expression was considered an independent prognostic factor associated with the poor survival of patients with TNBC. Overexpression of AHNAK in two TNBC cell lines, MDA-MB-231 and BT549, suppressed the in vitro TNBC cell proliferation and colony formation, and inhibited the in vivo TNBC xenograft growth and lung metastasis. The tumor suppressing effect of AHNAK in TNBC was associated with the AKT/MAPK signaling pathway and Wnt/β-catenin pathway. Consistent results were observed when AHNAK was knockdown in BT20 and MDA-MB-435 cells.

Conclusions

Taken together, our results suggest that AHNAK acts as a tumor suppressor that negatively regulates TNBC cell proliferation, TNBC xenograft growth and metastasis via different signaling pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-017-0522-4) contains supplementary material, which is available to authorized users.

Crystallographic anomalous diffraction data for the experimental phasing of two myelin proteins, gliomedin and periaxin

We present datasets that can be used for the experimental phasing of crystal structures of two myelin proteins. The structures were recently described in the articles “Periaxin and AHNAK nucleoprotein 2 form intertwined homodimers through domain swapping” (H. Han, P. Kursula, 2014) [1] and “The olfactomedin domain from gliomedin is a β-propeller with unique structural properties” (H. Han, P. Kursula, 2015) [2]. Crystals of periaxin were derivatized with tungsten and xenon prior to data collection, and diffraction data for these crystals are reported at 3 and 1 wavelengths, respectively. Crystallographic data for two different pressurizing times for xenon are provided. Gliomedin was derivatized with platinum, and data for single-wavelength anomalous dispersion are included. The data can be used to repeat the phasing experiments, to analyze heavy atom binding sites in proteins, as well as to optimize future derivatization experiments of protein crystals with these and other heavy-atom compounds.

Formation and carbon monoxide-dependent dissociation of Allochromatium vinosum cytochrome c' oligomers using domain-swapped dimers

The number of artificial protein supramolecules has been increasing; however, control of protein oligomer formation remains challenging. Cytochrome c' from Allochromatium vinosum (AVCP) is a homodimeric protein in its native form, where its protomer exhibits a four-helix bundle structure containing a covalently bound five-coordinate heme as a gas binding site. AVCP exhibits a unique reversible dimer-monomer transition according to the absence and presence of CO. Herein, domain-swapped dimeric AVCP was constructed and utilized to form a tetramer and high-order oligomers. The X-ray crystal structure of oxidized tetrameric AVCP consisted of two monomer subunits and one domain-swapped dimer subunit, which exchanged the region containing helices αA and αB between protomers. The active site structures of the domain-swapped dimer subunit and monomer subunits in the tetramer were similar to those of the monomer subunits in the native dimer. The subunit-subunit interactions at the interfaces of the domain-swapped dimer and monomer subunits in the tetramer were also similar to the subunit-subunit interaction in the native dimer. Reduced tetrameric AVCP dissociated to a domain-swapped dimer and two monomers upon CO binding. Without monomers, the domain-swapped dimers formed tetramers, hexamers, and higher-order oligomers in the absence of CO, whereas the oligomers dissociated to domain-swapped dimers in the presence of CO, demonstrating that the domain-swapped dimer maintains the CO-induced subunit dissociation behavior of native ACVP. These results suggest that protein oligomer formation may be controlled by utilizing domain swapping for a dimer-monomer transition protein.

Retracted Article: Self-association of L-periaxin occurs via its acidic domain and NLS2/NLS3, and affects its trafficking in RSC96 cells

Periaxin (PRX) protein was first identified in myelinating Schwann cells through the screening of cytoskeleton-associated proteins in peripheral nerve myelination.

Intermolecular disulfide bond in the dimerization of S-periaxin mediated by Cys88 and Cys139

Periaxin is expressed in mammalian Schwann cells and lens fiber cells, and has been identified in a screen for cytoskeleton-associated proteins. Charcot-Marie-Tooth 4F is caused by losses or mutations of theperiaxingene. Theperiaxingene encodes two protein isoforms, namely, L-periaxin and S-periaxin.S-periaxin contains 147 amino acid residues and has an N-terminal PDZ domain. In this paper, S-periaxin was reported to be homodimerized through the formation of intermolecular disulfide bonds with its Cys88 and Cys139 residues under mild oxidation conditions. The covalent dimer of S-periaxin was also observed by western blot analysis and bimolecular fluorescence complementation analyses. S-periaxin dimerization formation could be regulated by cellular redox fluctuations. These results offer a possible mechanism to the formation of periaxin complexes, improvement of complex stability, and establishment of a link between the extracellular matrix and the cytoskeleton.

Two independently folding units of Plasmodium profilin suggest evolution via gene fusion

Gene fusion is a common mechanism of protein evolution that has mainly been discussed in the context of multidomain or symmetric proteins. Less is known about fusion of ancestral genes to produce small single-domain proteins. Here, we show with a domain-swapped mutant Plasmodium profilin that this small, globular, apparently single-domain protein consists of two foldons. The separation of binding sites for different protein ligands in the two halves suggests evolution via an ancient gene fusion event, analogous to the formation of multidomain proteins. Finally, the two fragments can be assembled together after expression as two separate gene products. The possibility to engineer both domain-swapped dimers and half-profilins that can be assembled back to a full profilin provides perspectives for engineering of novel protein folds, e.g., with different scaffolding functions.

L-periaxin interacts with S-periaxin through its PDZ domain

Periaxin was first identified as a protein in myelinating Schwann cells through a screen of novel cytoskeleton-associated proteins in peripheral nerve myelination. The periaxin gene encodes two isoforms, namely, L- and S-periaxin, which are 1461 and 147 residues in size, respectively. Several loss-of-function mutations linked to autosomal recessive Dejerine-Sottas neuropathy and demyelinating Charcot-Marie-Tooth disease in periaxin have been described. In this study, the colocolization of L- and S-periaxin in the cytoplasm of RSC96 cells was found by immunofluorescence assays. The interaction between these two isoforms was confirmed by co-immunoprecipitation, fluorescence complementation experiment, and GST pull-down assay. Results also showed that the two periaxin isoforms interacted in the cytoplasm through the PDZ domain, and their interaction prevented the homodimerization of L-periaxin. S-periaxin may regulate the function of L-periaxin in Schwann cells.

Change in structure and ligand binding properties of hyperstable cytochrome c555 from Aquifex aeolicus by domain swapping

Cytochrome c555 from hyperthermophilic bacteria Aquifex aeolicus (AA cyt c555 ) is a hyperstable protein belonging to the cyt c protein family, which possesses a unique long 310 -α-310 helix containing the heme-ligating Met61. Herein, we show that AA cyt c555 forms dimers by swapping the region containing the extra 310 -α-310 helix and C-terminal α-helix. The asymmetric unit of the crystal of dimeric AA cyt c555 contained two dimer structures, where the structure of the hinge region (Val53-Lys57) was different among all four protomers. Dimeric AA cyt c555 dissociated to monomers at 92 ± 1°C according to DSC measurements, showing that the dimer was thermostable. According to CD measurements, the secondary structures of dimeric AA cyt c555 were maintained at pH 2.2-11.0. CN(-) and CO bound to dimeric AA cyt c555 in the ferric and ferrous states, respectively, owing to the flexibility of the hinge region close to Met61 in the dimer, whereas these ligands did not bind to the monomer under the same conditions. In addition, CN(-) and CO bound to the oxidized and reduced dimer at neutral pH and a wide range of pH (pH 2.2-11.0), respectively, in a wide range of temperature (25-85°C), owing to the thermostability and pH tolerance of the dimer. These results show that the ligand binding character of hyperstable AA cyt c555 changes upon dimerization by domain swapping.

AHNAK: the giant jack of all trades

The nucleoprotein AHNAK is an unusual and somewhat mysterious scaffolding protein characterised by its large size of approximately 700 kDa. Several aspects of this protein remain uncertain, including its exact molecular function and regulation on both the gene and protein levels. Various studies have attempted to annotate AHNAK and, notably, protein interaction and expression analyses have contributed greatly to our current understanding of the protein. The implicated biological processes are, however, very diverse, ranging from a role in the formation of the blood-brain barrier, cell architecture and migration, to the regulation of cardiac calcium channels and muscle membrane repair. In addition, recent evidence suggests that AHNAK might be yet another accomplice in the development of tumour metastasis. This review will discuss the different functional roles of AHNAK, highlighting recent advancements that have added foundation to the proposed roles while identifying ties between them. Implications for related fields of research are noted and suggestions for future research that will assist in unravelling the function of AHNAK are offered.