Two Crystal Structures of Bombyx mori Lipoprotein 3 - Structural Characterization of a New 30-kDa Lipoprotein Family Member

The 30-kDa family of lipoproteins from insect hemolymph has been the focus of a number of studies over the last few years. Recently, four crystal structures of Bombyx mori lipoprotein 7 have been determined. Here we report two crystal structures of another member of the 30-kDa lipoprotein family, Bombyx mori lipoprotein 3 (Bmlp3). The protein was isolated from its natural source, mulberry silkworm hemolymph. It crystallized in two different crystal forms, Bmlp3-p21 (space group P2₁) and Bmlp3-c2 (space group C2). The crystal structures were solved by molecular replacement using the coordinates of Bmlp7 as a starting model. The crystals of Bmlp3-p21 diffracted X-rays to 2.4 Å resolution and of Bmlp3-c2 to 2.1 Å resolution. Bmlp3 has an overall fold characteristic of 30-kDa lipoproteins, with a VHS-type N-terminal domain and β-trefoil C-terminal domain. Structural comparison of Bmlp3 and Bmlp7 shows that the loops present in the C-terminal domain are flexible and participate in dimer formation. Additionally, new putative binding sites of Bmlp3 have been analyzed in detail and the electrostatic potential of the protein surface at physiological pH 7.4 conditions has been calculated. The results of these calculations are the starting point for an explanation of the recently reported cell-penetrating properties of the 30-kDa lipoproteins.     

Crystal structure of the 30 K protein from the silkworm Bombyx mori reveals a new member of the β-trefoil superfamily

The hemolymph of the fifth instar larvae of the silkworm Bombyx mori contains a group of homologous proteins with a molecular weight of approximately 30 kDa, termed B. mori low molecular weight lipoproteins (Bmlps), which account for about 5% of the total plasma proteins. These so-called "30 K proteins" have been reported to be involved in the innate immune response and transportation of lipid and/or sugar. To elucidate their molecular functions, we determined the crystal structure of a 30 K protein, Bmlp7, at 1.91?. It has two distinct domains: an all-α N-terminal domain (NTD) and an all-β C-terminal domain (CTD) of the β-trefoil fold. Comparative structural analysis indicates that Bmlp7 represents a new family, adding to the 14 families currently identified, of the β-trefoil superfamily. Structural comparison and simulation suggest that the NTD has a putative lipid-binding cavity, whereas the CTD has a potential sugar-binding site. However, we were unable to detect the binding of either lipid or sugar. Therefore, further investigations are needed to characterize the molecular function of this protein.     

Overexpression and functional characterization of an Aspergillus niger phytase in the fat body of transgenic silkworm,Bombyx mori

In a previous study, we isolated 1,119 bp of upstream promoter sequence from Bmlp3, a gene encoding a member of the silkworm 30 K storage protein family, and demonstrated that it was sufficient to direct fat body-specific expression of a reporter gene in a transgenic silkworm, thus highlighting the potential use of this promoter for both functional genomics research and biotechnology applications. To test whether the Bmlp3 promoter can be used to produce recombinant proteins in the fat body of silkworm pupae, we generated a transgenic line of Bombyx mori which harbors a codon-optimized Aspergillus niger phytase gene (phyA) under the control of the Bmlp3 promoter. Here we show that the Bmlp3 promoter drives high levels of phyA expression in the fat body, and that the recombinant phyA protein is highly active (99.05 and 54.80 U/g in fat body extracts and fresh pupa, respectively). We also show that the recombinant phyA has two optimum pH ranges (1.5–2.0 and 5.5–6.0), and two optimum temperatures (55 and 37 °C). The activity of recombinant phyA was lost after high-temperature drying, but treating with boiling water was less harmful, its residual activity was approximately 84 % of the level observed in untreated samples. These results offer an opportunity not only for better utilization of large amounts of silkworm pupae generated during silk production, but also provide a novel method for mass production of low-cost recombinant phytase using transgenic silkworms.     

The promoter of Bmlp3 gene can direct fat body-specific expression in the transgenic silkworm, Bombyx mori

The fat body plays multiple, crucial roles in the life of silkworms. Targeted expression of transgenes in the fat body of the silkworm, Bombyx mori, is important not only for clarifying the function of endogenous genes expressed in this tissue, but also for producing valuable recombinant proteins. However, fat body-specific gene expression remains difficult due to a lack of suitable tissue-specific promoters. Here we report the isolation of the fat body-specific promoter of Bmlp3, a member of the 30K protein family of silkworms. The 1.1 kb fragment from ?374 to +738 of Bmlp3 displayed strong promoter activity in the cell lines BmE and Spli-221. In transgenic silkworms, a DsRed reporter gene controlled by the 1.1 kb Bmlp3 promoter fragment was expressed specifically in the fat body in a stage-specific pattern that was nearly identical to the endogenous Bmlp3 gene. We conclude that the 1.1 kb Bmlp3 promoter fragment is sufficient to direct tissue- and stage-specific expression of transgenes in the fat body of silkworms, highlighting the potential use of this promoter for both functional genomics research and biotechnology applications.     

Protective Effect of the Silkworm Protein 30Kc6 on Human Vascular Endothelial Cells Damaged by Oxidized Low Density Lipoprotein (Ox-LDL)

Although the 30K family proteins are important anti-apoptotic molecules in silkworm hemolymph, the underlying mechanism remains to be investigated. This is especially the case in human vascular endothelial cells (HUVECs). In this study, a 30K protein, 30Kc6, was successfully expressed and purified using the Bac-to-Bac baculovirus expression system in silkworm cells. Furthermore, the 30Kc6 expressed in Escherichia coli was used to generate a polyclonal antibody. Western blot analysis revealed that the antibody could react specifically with the purified 30Kc6 expressed in silkworm cells. The In vitro cell apoptosis model of HUVEC that was induced by oxidized low density lipoprotein (Ox-LDL) and in vivo atherosclerosis rabbit model were constructed and were employed to analyze the protective effects of the silkworm protein 30Kc6 on these models. The results demonstrated that the silkworm protein 30Kc6 significantly enhanced the cell viability in HUVEC cells treated with Ox-LDL, decreased the degree of DNA fragmentation and markedly reduced the level of 8-isoprostane. This could be indicative of the silkworm protein 30Kc6 antagonizing the Ox-LDL-induced cell apoptosis by inhibiting the intracellular reactive oxygen species (ROS) generation. Furthermore, Ox-LDL activated the cell mitogen activated protein kinases (MAPK), especially JNK and p38. As demonstrated with Western analysis, 30Kc6 inhibited Ox-LDL-induced cell apoptosis in HUVEC cells by preventing the MAPK signaling pathways. In vivo data have demonstrated that oral feeding of the silkworm protein 30Kc6 dramatically improved the conditions of the atherosclerotic rabbits by decreasing serum levels of total triglyceride (TG), high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C) and total cholesterol (TC). Furthermore, 30Kc6 alleviated the extent of lesions in aorta and liver in the atherosclerotic rabbits. These data are not only helpful in understanding the anti-apoptotic mechanism of the 30K family proteins, but also provide important information on prevention and treatment of human cardiovascular diseases.     

Lipopeptides as main ingredients for inhibition of fungal phytopathogens by Bacillus subtilis/amyloliquefaciens

Some isolates of the Bacillus subtilis/amyloliquefaciens species are known for their plant protective activity against fungal phytopathogens. It is notably due to their genetic potential to form an impressive array of antibiotics including non-ribosomal lipopeptides (LPs). In the work presented here, we wanted to gain further insights into the relative role of these LPs in the global antifungal activity of B. subtilis/amyloliquefaciens. To that end, a comparative study was conducted involving multiple strains that were tested against four different phytopathogens. We combined various approaches to further exemplify that secretion of those LPs is a crucial trait in direct pathogen ward off and this can actually be generalized to all members of these species. Our data illustrate that for each LP family, the fungitoxic activity varies in function of the target species and that the production of iturins and fengycins is modulated by the presence of pathogens. Our data on the relative involvement of these LPs in the biocontrol activity and modulation of their production are discussed in the context of natural conditions in the rhizosphere.     

Nonribosomal Peptide Synthase Gene Clusters for Lipopeptide Biosynthesis in Bacillus subtilis 916 and Their Phenotypic Functions

Bacillus cyclic lipopeptides (LPs) have been well studied for their phytopathogen-antagonistic activities. Recently, research has shown that these LPs also contribute to the phenotypic features of Bacillus strains, such as hemolytic activity, swarming motility, biofilm formation, and colony morphology. Bacillus subtilis 916 not only coproduces the three families of well-known LPs, i.e., surfactins, bacillomycin Ls (iturin family), and fengycins, but also produces a new family of LP called locillomycins. The genome of B. subtilis 916 contains four nonribosomal peptide synthase (NRPS) gene clusters, srf, bmy, fen, and loc, which are responsible for the biosynthesis of surfactins, bacillomycin Ls, fengycins, and locillomycins, respectively. By studying B. subtilis 916 mutants lacking production of one, two, or three LPs, we attempted to unveil the connections between LPs and phenotypic features. We demonstrated that bacillomycin Ls and fengycins contribute mainly to antifungal activity. Although surfactins have weak antifungal activity in vitro, the strain mutated in srfAA had significantly decreased antifungal activity. This may be due to the impaired productions of fengycins and bacillomycin Ls. We also found that the disruption of any LP gene cluster other than fen resulted in a change in colony morphology. While surfactins and bacillomycin Ls play very important roles in hemolytic activity, swarming motility, and biofilm formation, the fengycins and locillomycins had little influence on these phenotypic features. In conclusion, B. subtilis 916 coproduces four families of LPs which contribute to the phenotypic features of B. subtilis 916 in an intricate way.     

On the structure and chromosome location of the 72- and 92-kDa human type IV collagenase genes.

The 72- and 92-kDa type IV collagenases are members of a group of secreted zinc metalloproteases. Two members of this family, collagenase and stromelysin, have previously been localized to the long arm of chromosome 11. Here we assign both of the two type IV collagenase genes to human chromosome 16. By sequencing, the 72-kDa gene is shown to consist of 13 exons, 3 more than have been reported for the other members of this gene family. The extra exons encode the amino acids of the fibronectin-like domain which has so far been found in only the 72- and 92-kDa type IV collagenase. The evolutionary relationship among the members of this gene family is discussed.     

Structural and functional features of the NAD(P) dependent Gfo/Idh/MocA protein family oxidoreductases

The Gfo/Idh/MocA protein family contains a number of different proteins, which almost exclusively consist of NAD(P)‐dependent oxidoreductases that have a diverse set of substrates, typically pyranoses. In this study, to clarify common structural features that would contribute to their function, the available crystal structures of the members of this family have been analyzed. Despite a very low sequence identity, the central features of the three‐dimensional structures of the proteins are surprisingly similar. The members of the protein family have a two‐domain structure consisting of a N‐terminal nucleotide‐binding domain and a C‐terminal α/β‐domain. The C‐terminal domain contributes to the substrate binding and catalysis, and contains a βα‐motif with a central α‐helix carrying common essential amino acid residues. The β‐sheet of the α/β‐domain contributes to the oligomerization in most of the proteins in the family.     

Combination of classical and statistical approaches to enhance the fermentation conditions and increase the yield of Lipopeptide(s) by Pseudomonas sp. OXDC12: its partial purification and determining antifungal property

Around 200 different lipopeptides (LPs) have been identified to date, most of which are produced via Bacillus and Pseudomonas species. The clinical nature of the lipopeptide (LP) has led to a big surge in its research. They show antimicrobial and antitumor activities due to which mass-scale production and purification of LPs are beneficial. Response surface methodology (RSM) approach has emerged as an alternative in the field of computational biology for optimizing the reaction parameters using statistical models. In the present study, Pseudomonas sp. strain OXDC12 was used for production and partial purification of LPs using Thin Layer Chromatography (TLC). The main goal of the study was to increase the overall yield of LPs by optimizing the different variables in the fermentation broth. This was achieved using a combination of one factor at a time (OFAT) and response surface methodology (RSM) approaches. OFAT technique was used to optimize the necessary parameters and was followed by the creation of statistical models (RSM) to optimize the remaining variables. Maximum mycelial growth inhibition (%) against the fungus Mucor sp. was 61.3% for LP. Overall, the combination of both OFAT and RSM helped in increasing the LPs yield by 3 folds from 367mg/L to 1169mg/L.     

Structural Insights into the Anti-HIV Activity of the Oscillatoria agardhii Agglutinin Homolog Lectin Family

Oscillatoria agardhii agglutinin homolog (OAAH) proteins belong to a recently discovered lectin family. All members contain a sequence repeat of ∼66 amino acids, with the number of repeats varying among different family members. Apart from data for the founding member OAA, neither three-dimensional structures, information about carbohydrate binding specificities, nor antiviral activity data have been available up to now for any other members of the OAAH family. To elucidate the structural basis for the antiviral mechanism of OAAHs, we determined the crystal structures of Pseudomonas fluorescens and Myxococcus xanthus lectins. Both proteins exhibit the same fold, resembling the founding family member, OAA, with minor differences in loop conformations. Carbohydrate binding studies by NMR and x-ray structures of glycan-lectin complexes reveal that the number of sugar binding sites corresponds to the number of sequence repeats in each protein. As for OAA, tight and specific binding to α3,α6-mannopentaose was observed. All the OAAH proteins described here exhibit potent anti-HIV activity at comparable levels. Altogether, our results provide structural details of the protein-carbohydrate interaction for this novel lectin family and insights into the molecular basis of their HIV inactivation properties.     

Structural and Functional Divergence within the Dim1/KsgA Family of rRNA Methyltransferases

The enzymes of the KsgA/Dim1 family are universally distributed throughout all phylogeny; however, structural and functional differences are known to exist. The well-characterized function of these enzymes is to dimethylate two adjacent adenosines of the small ribosomal subunit in the normal course of ribosome maturation, and the structures of KsgA from Escherichia coli and Dim1 from Homo sapiens and Plasmodium falciparum have been determined. To this point, no examples of archaeal structures have been reported. Here, we report the structure of Dim1 from the thermophilic archaeon Methanocaldococcus jannaschii. While it shares obvious similarities with the bacterial and eukaryotic orthologs, notable structural differences exist among the three members, particularly in the C-terminal domain. Previous work showed that eukaryotic and archaeal Dim1 were able to robustly complement for KsgA in E. coli. Here, we repeated similar experiments to test for complementarity of archaeal Dim1 and bacterial KsgA in Saccharomyces cerevisiae. However, neither the bacterial nor the archaeal ortholog could complement for the eukaryotic Dim1. This might be related to the secondary, non-methyltransferase function that Dim1 is known to play in eukaryotic ribosomal maturation. To further delineate regions of the eukaryotic Dim1 critical to its function, we created and tested KsgA/Dim1 chimeras. Of the chimeras, only one constructed with the N-terminal domain from eukaryotic Dim1 and the C-terminal domain from archaeal Dim1 was able to complement, suggesting that eukaryotic-specific Dim1 function resides in the N-terminal domain also, where few structural differences are observed between members of the KsgA/Dim1 family. Future work is required to identify those determinants directly responsible for Dim1 function in ribosome biogenesis. Finally, we have conclusively established that none of the methyl groups are critically important to growth in yeast under standard conditions at a variety of temperatures.     

Apo and Calcium-Bound Crystal Structures of Cytoskeletal Protein Alpha-14 Giardin (Annexin E1) from the Intestinal Protozoan Parasite Giardia lamblia

Alpha-14 giardin (annexin E1), a member of the alpha giardin family of annexins, has been shown to localize to the flagella of the intestinal protozoan parasite Giardia lamblia. Alpha giardins show a common ancestry with the annexins, a family of proteins most of which bind to phospholipids and cellular membranes in a Ca²⁺-dependent manner and are implicated in numerous membrane-related processes including cytoskeletal rearrangements and membrane organization. It has been proposed that alpha-14 giardin may play a significant role during the cytoskeletal rearrangement during differentiation of Giardia. To gain a better understanding of alpha-14 giardin's mode of action and its biological role, we have determined the three-dimensional structure of alpha-14 giardin and its phospholipid-binding properties. Here, we report the apo crystal structure of alpha-14 giardin determined in two different crystal forms as well as the Ca²⁺-bound crystal structure of alpha-14 giardin, refined to 1.9, 1.6 and 1.65 Å, respectively. Although the overall fold of alpha-14 giardin is similar to that of alpha-11 giardin, multiwavelength anomalous dispersion phasing was required to solve the alpha-14 giardin structure, indicating significant structural differences between these two members of the alpha giardin family. Unlike most annexin structures, which typically possess N-terminal domains, alpha-14 giardin is composed of only a core domain, followed by a C-terminal extension that may serve as a ligand for binding to cytoskeletal protein partners in Giardia. In the Ca²⁺-bound structure we detected five bound calcium ions, one of which is a novel, highly coordinated calcium-binding site not previously observed in annexin structures. This novel high-affinity calcium-binding site is composed of seven protein donor groups, a feature rarely observed in crystal structures. In addition, phospholipid-binding assays suggest that alpha-14 giardin exhibits calcium-dependent binding to phospholipids that coordinate cytoskeletal disassembly/assembly during differentiation of the parasite.     

Crystal structures of sialyltransferase from Photobacterium damselae

Sialyltransferase structures fall into either GT-A or GT-B glycosyltransferase fold. Some sialyltransferases from the Photobacterium genus have been shown to contain an additional N-terminal immunoglobulin (Ig)-like domain. Photobacterium damselae α2–6-sialyltransferase has been used efficiently in enzymatic and chemoenzymatic synthesis of α2–6-linked sialosides. Here we report three crystal structures of this enzyme. Two structures with and without a donor substrate analog CMP-3F(a)Neu5Ac contain an immunoglobulin (Ig)-like domain and adopt the GT-B sialyltransferase fold. The binary structure reveals a non-productive pre-Michaelis complex, which are caused by crystal lattice contacts that prevent the large conformational changes. The third structure lacks the Ig-domain. Comparison of the three structures reveals small inherent flexibility between the two Rossmann-like domains of the GT-B fold.     

Bacterial Pleckstrin Homology Domains: A Prokaryotic Origin for the PH Domain

Pleckstrin homology (PH) domains have been identified only in eukaryotic proteins to date. We have determined crystal structures for three members of an uncharacterized protein family (Pfam PF08000), which provide compelling evidence for the existence of PH-like domains in bacteria (PHb). The first two structures contain a single PHb domain that forms a dome-shaped, oligomeric ring with C₅ symmetry. The third structure has an additional helical hairpin attached at the C-terminus and forms a similar but much larger ring with C₁₂ symmetry. Thus, both molecular assemblies exhibit rare, higher-order, cyclic symmetry but preserve a similar arrangement of their PHb domains, which gives rise to a conserved hydrophilic surface at the intersection of the β-strands of adjacent protomers that likely mediates protein-protein interactions. As a result of these structures, additional families of PHb domains were identified, suggesting that PH domains are much more widespread than originally anticipated. Thus, rather than being a eukaryotic innovation, the PH domain superfamily appears to have existed before prokaryotes and eukaryotes diverged.     

Functional dynamics of SARS-CoV-2 3C-like protease as a member of clan PA

SARS-CoV-2 3C-like protease (3CL^pro), a potential therapeutic target for COVID-19, consists of a chymotrypsin fold and a C-terminal α-helical domain (domain III), the latter of which mediates dimerization required for catalytic activation. To gain further understanding of the functional dynamics of SARS-CoV-2 3CL^pro, this review extends the scope to the comparative study of many crystal structures of proteases having the chymotrypsin fold (clan PA of the MEROPS database). First, the close correspondence between the zymogen-enzyme transformation in chymotrypsin and the allosteric dimerization activation in SARS-CoV-2 3CL^pro is illustrated. Then, it is shown that the 3C-like proteases of family Coronaviridae (the protease family C30), which are closely related to SARS-CoV-2 3CL^pro, have the same homodimeric structure and common activation mechanism via domain III mediated dimerization. The survey extended to order Nidovirales reveals that all 3C-like proteases belonging to Nidovirales have domain III, but with various chain lengths, and 3CL^pro of family Mesoniviridae (family C107) has the same homodimeric structure as that of C30, even though they have no sequence similarity. As a reference, monomeric 3C proteases belonging to the more distant family Picornaviridae (family C3) lacking domain III are compared with C30, and it is shown that the 3C proteases are rigid enough to maintain their structures in the active state.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12551-022-01020-x.     

The Serine Protease Domain of MASP-3: Enzymatic Properties and Crystal Structure in Complex with Ecotin

Mannan-binding lectin (MBL), ficolins and collectin-11 are known to associate with three homologous modular proteases, the MBL-Associated Serine Proteases (MASPs). The crystal structures of the catalytic domains of MASP-1 and MASP-2 have been solved, but the structure of the corresponding domain of MASP-3 remains unknown. A link between mutations in the MASP1/3 gene and the rare autosomal recessive 3MC (Mingarelli, Malpuech, Michels and Carnevale,) syndrome, characterized by various developmental disorders, was discovered recently, revealing an unexpected important role of MASP-3 in early developmental processes. To gain a first insight into the enzymatic and structural properties of MASP-3, a recombinant form of its serine protease (SP) domain was produced and characterized. The amidolytic activity of this domain on fluorescent peptidyl-aminomethylcoumarin substrates was shown to be considerably lower than that of other members of the C1r/C1s/MASP family. The E. coli protease inhibitor ecotin bound to the SP domains of MASP-3 and MASP-2, whereas no significant interaction was detected with MASP-1, C1r and C1s. A tetrameric complex comprising an ecotin dimer and two MASP-3 SP domains was isolated and its crystal structure was solved and refined to 3.2 Å. Analysis of the ecotin/MASP-3 interfaces allows a better understanding of the differential reactivity of the C1r/C1s/MASP protease family members towards ecotin, and comparison of the MASP-3 SP domain structure with those of other trypsin-like proteases yields novel hypotheses accounting for its zymogen-like properties in vitro.     

Discovery,Primary, and Crystal Structures and Capacitation-related Properties of a Prostate-derived Heparin-binding Protein WGA16 from Boar Sperm

Mammalian sperm acquire fertility through a functional maturation process called capacitation, where sperm membrane molecules are drastically remodeled. In this study, we found that a wheat germ agglutinin (WGA)-reactive protein on lipid rafts, named WGA16, is removed from the sperm surface on capacitation. WGA16 is a prostate-derived seminal plasma protein that has never been reported and is deposited on the sperm surface in the male reproductive tract. Based on protein and cDNA sequences for purified WGA16, it is a homologue of human zymogen granule protein 16 (ZG16) belonging to the Jacalin-related lectin (JRL) family in crystal and primary structures. A glycan array shows that WGA16 binds heparin through a basic patch containing Lys-53/Lys-73 residues but not the conventional lectin domain of the JRL family. WGA16 is glycosylated, contrary to other ZG16 members, and comparative mass spectrometry clearly shows its unique N-glycosylation profile among seminal plasma proteins. It has exposed GlcNAc and GalNAc residues without additional Gal residues. The GlcNAc/GalNAc residues can work as binding ligands for a sperm surface galactosyltransferase, which actually galactosylates WGA16 in situ in the presence of UDP-Gal. Interestingly, surface removal of WGA16 is experimentally induced by either UDP-Gal or heparin. In the crystal structure, N-glycosylated sites and a potential heparin-binding site face opposite sides. This geography of two functional sites suggest that WGA16 is deposited on the sperm surface through interaction between its N-glycans and the surface galactosyltransferase, whereas its heparin-binding domain may be involved in binding to sulfated glycosaminoglycans in the female tract, enabling removal of WGA16 from the sperm surface.     

Structure of the dimeric autoinhibited conformation of DAPK2, a pro-apoptotic protein kinase

The death-associated protein kinase (DAPK) family has been characterized as a group of pro-apoptotic serine/threonine kinases that share specific structural features in their catalytic kinase domain. Two of the DAPK family members, DAPK1 and DAPK2, are calmodulin-dependent protein kinases that are regulated by oligomerization, calmodulin binding, and autophosphorylation. In this study, we have determined the crystal and solution structures of murine DAPK2 in the presence of the autoinhibitory domain, with and without bound nucleotides in the active site. The crystal structure shows dimers of DAPK2 in a conformation that is not permissible for protein substrate binding. Two different conformations were seen in the active site upon the introduction of nucleotide ligands. The monomeric and dimeric forms of DAPK2 were further analyzed for solution structure, and the results indicate that the dimers of DAPK2 are indeed formed through the association of two apposed catalytic domains, as seen in the crystal structure. The structures can be further used to build a model for DAPK2 autophosphorylation and to compare with closely related kinases, of which especially DAPK1 is an actively studied drug target. Our structures also provide a model for both homodimerization and heterodimerization of the catalytic domain between members of the DAPK family. The fingerprint of the DAPK family, the basic loop, plays a central role in the dimerization of the kinase domain.