Calpain-mediated regulation of platelet signaling pathways

PURPOSE OF REVIEW

There is considerable interest in understanding the function and mechanism of calpains in platelet aggregation, spreading, and granular secretion pathways. Recent insights from the calpain-1 knockout platelets suggest a pivotal role of these cysteine proteases in the regulation of outside-in signaling, aggregation, and clot retraction.

RECENT FINDINGS

The calpain-1 knockout mouse provided direct evidence for the role of calpain-1 in platelet aggregation and clot retraction. Reduced tyrosine phosphorylation of platelet proteins correlated with reduced platelet aggregation and clot retraction. Future investigations of the mechanism of platelet defects in calpain-1 null mice may unveil the physiological functions of this important and elusive protease in mammalian cells.

SUMMARY

This review focuses on the role of calpains in platelets with a particular emphasis on recent findings in calpain-1 null platelets. Previous studies used synthetic inhibitors to study the role of calpains in platelet function yielding useful information about the identification of calpain substrates. The development of calpain-1 null mice demonstrated that calpain-1 plays an important function in the regulation of platelet aggregation and clot retraction. Since the combined deletion of calpain-1 and calpain-2 genes results in embryonic lethality, the calpain-1 null mouse remains the only experimental model available to study the physiological role of calpains in mammalian cells.

Purification of the NF2 tumor suppressor protein from human erythrocytes

BACKGROUND

Neurofibromatosis type 2 (NF2) is an autosomal dominant disease predisposing individuals to the risk of developing tumors of cranial and spinal nerves. The NF2 tumor suppressor protein, known as Merlin/Schwanomin, is a member of the protein 4.1 superfamily that function as links between the cytoskeleton and the plasma membrane.

METHODS

Upon selective extraction of membrane-associated proteins from erythrocyte plasma membrane (ghosts) using low ionic strength solution, the bulk of NF2 protein remains associated with the spectrin-actin depleted inside-out-vesicles. Western blot analysis showed a approximately 70 kDa polypeptide in the erythrocyte plasma membrane. Furthermore, quantitative removal of NF2 protein from the inside-out-vesicles was achieved using 1.0 M potassium iodide, a treatment known to remove tightly-bound peripheral membrane proteins.

RESULTS

These results suggest a novel mode of NF2 protein association with the erythrocyte membrane that is distinct from the known membrane interactions of protein 4.1. Based on these biochemical properties, several purification strategies were devised to isolate native NF2 protein from human erythrocyte ghosts. Using purified and recombinant NF2 protein as internal standards, we quantified approximately 41-65,000 molecules of NF2 protein per erythrocyte.

CONCLUSION

We provide evidence for the presence of NF2 protein in the human erythrocyte membrane. The identification of NF2 protein in the human erythrocyte membrane will make it feasible to discover novel interactions of NF2 protein utilizing powerful techniques of erythrocyte biochemistry and genetics in mammalian cells.

Combined deletion of mouse dematin-headpiece and beta-adducin exerts a novel effect on the spectrin-actin junctions leading to erythrocyte fragility and hemolytic anemia

Dematin and adducin are actin-binding proteins of the erythrocyte "junctional complex." Individually, they exert modest effects on erythrocyte shape and membrane stability, and their homologues are expressed widely in non-erythroid cells. Here we report generation and characterization of double knock-out mice lacking beta-adducin and the headpiece domain of dematin. The combined mutations result in altered erythrocyte morphology, increased membrane instability, and severe hemolysis. Peripheral blood analysis shows evidence of severe hemolytic anemia with reduced number of erythrocytes/hematocrit/hemoglobin and an approximately 12-fold increase in the number of circulating reticulocytes. The presence of a variety of misshapen and fragmented erythrocytes correlates with increased osmotic fragility and reduced in vivo life span. Despite the apparently normal protein composition of the mutant erythrocyte membrane, the retention of the spectrin-actin complex in the membrane under low ionic strength conditions is significantly reduced by the double mutation. Atomic force microscopy reveals an increase in grain size and a decrease in filament number of the mutant membrane cytoskeleton, although the volume parameter is similar to wild type erythrocytes. Aggregated, disassembled, and irregular features are visualized in the mutant membrane, consistent with the presence of large protein aggregates. Importantly, purified dematin binds to the stripped inside-out vesicles in a saturable manner, and dematin-membrane binding is abolished upon pretreatment of membrane vesicles with trypsin. Together, these results reveal an essential role of dematin and adducin in the maintenance of erythrocyte shape and membrane stability, and they suggest that the dematin-membrane interaction could link the junctional complex to the plasma membrane in erythroid cells.

μ-Calpain is essential for postmortem proteolysis of muscle proteins1,2

The objective of this investigation was to test the hypothesis that -calpain is largely responsible for postmortem proteolysis of muscle proteins. To accomplish this objective, we compared proteolysis of known muscle proteins in muscles of wild type and micro-calpain knockout mice during postmortem storage. Knockout mice (n = 6) were killed along with control mice (n = 6). Hind limbs were removed and stored at 4 degrees C. Muscles were dissected at 0, 1, and 3d postmortem and subsequently analyzed for degradation of nebulin, dystrophin, metavinculin, vinculin, desmin, and troponin T. In a separate experiment, hind limb muscles from knockout (n = 4) and control mice (n = 4) were analyzed at 0, 1, and 3 d postmortem using casein zymography to confirm that mu-calpain activity was knocked out in muscle and to determine whether or not m-calpain is activated in murine postmortem muscle. Cumulatively, the results of the first experiment indicated that postmortem proteolysis was largely inhibited in micro-calpain knockout mice. The results of the second experiment established the absence of micro-calpain in the muscle tissue of knockout mice and confirmed the results of an earlier study that m-calpain is active in postmortem murine muscle. The results of the current study show that even in a species in which m-calpain is activated to some extent postmortem, micro-calpain is largely responsible for postmortem proteolysis. This observation excludes a major role for any of the other members of the calpain family or any other proteolytic system in postmortem proteolysis of muscle proteins. Therefore, understanding the regulation of micro-calpain in postmortem muscle should be the focus of further research on postmortem proteolysis and tenderization of meat.

Transport of PIP3 by GAKIN, a kinesin-3 family protein, regulates neuronal cell polarity

Phosphatidylinositol-(3,4,5)-trisphosphate (PIP₃), a product of phosphatidylinositol 3-kinase, is an important second messenger implicated in signal transduction and membrane transport. In hippocampal neurons, the accumulation of PIP₃ at the tip of neurite initiates the axon specification and neuronal polarity formation. We show that guanylate kinase–associated kinesin (GAKIN), a kinesin-like motor protein, directly interacts with a PIP₃-interacting protein, PIP₃BP, and mediates the transport of PIP₃-containing vesicles. Recombinant GAKIN and PIP₃BP form a complex on synthetic liposomes containing PIP₃ and support the motility of the liposomes along microtubules in vitro. In PC12 cells and cultured hippocampal neurons, transport activity of GAKIN contributes to the accumulation of PIP₃ at the tip of neurites. In hippocampal neurons, altered accumulation of PIP₃ by overexpression of GAKIN constructs led to the loss of the axonally differentiated neurites. Together, these results suggest that, in neurons, the GAKIN–PIP₃BP complex transports PIP₃ to the neurite ends and regulates neuronal polarity formation.

Host receptors in malaria merozoite invasion

The clinical manifestations of Plasmodium falciparum malaria are directly linked to the blood stage of the parasite life cycle. At the blood stage, the circulating merozoites invade erythrocytes via a specific invasion pathway often identified with its dependence or independence on sialic acid residues of the host receptor. The invasion process involves multiple receptor-ligand interactions that mediate a complex series of events in a period of approximately 1 min. Although the mechanism by which merozoites invade erythrocytes is not fully understood, recent advances have put a new perspective on the importance of developing a multivalent blood stage-malaria vaccine. In this review, we highlight the role of currently identified host invasion receptors in blood-stage malaria.

Novel Peptidomimetic Cysteine Protease Inhibitors as Potential Antimalarial Agents

The synthesis of a new class of peptidomimetics 1a-j, based on a 1,4-benzodiazepine scaffold and on a C-terminal aspartyl aldehyde building block, is described. Compounds 1a-j provided significant inhibitory activity against falcipains 2A and 2B (FP-2A and FP-2B), two cysteine proteases from Plasmodium falciparum.

Cloning and characterization of Plasmodium falciparum cysteine protease, falcipain-2B

The gene for malaria parasite cysteine protease falcipain-2B has been isolated from the Plasmodium falciparum genomic DNA. Falcipain-2B gene is located adjacent to the falcipain-2A gene on chromosome 11, and the two enzymes show extensive sequence identity at the amino acid level. Using reverse transcribed polymerase chain reaction (RT-PCR), the transcript of falcipain-2B was detected at the trophozoite stage of P. falciparum in human erythrocytes. Recombinant falcipain-2B protein expressed in bacteria exhibits protease activity as established by the cleavage of fluorescent peptide substrate as well as in-gel gelatin zymography. Importantly, the recombinant falcipain-2B cleaved host ankyrin but not protein 4.1 as assessed by the erythrocyte inside-out-vesicle assay in vitro. Notwithstanding its predicted hemoglobinase function, the P. falciparum falcipain-2B may contribute and orchestrate selective proteolytic events during the exit of malaria parasite from human red blood cells.

Calpain is required for normal osteoclast function and is down-regulated by calcitonin

Osteoclast motility is thought to depend on rapid podosome assembly and disassembly. Both mu-calpain and m-calpain, which promote the formation and disassembly of focal adhesions, were observed in the podosome belt of osteoclasts. Calpain inhibitors disrupted the podosome belt, blocked the constitutive cleavage of the calpain substrates filamin A, talin, and Pyk2, which are enriched in the podosome belt, induced osteoclast retraction, and reduced osteoclast motility and bone resorption. The motility and resorbing activity of mu-calpain(-/-) osteoclast-like cells were also reduced, indicating that mu-calpain is required for normal osteoclast activity. Histomorphometric analysis of tibias from mu-calpain(-/-) mice revealed increased osteoclast numbers and decreased trabecular bone volume that was apparent at 10 weeks but not at 5 weeks of age. In vitro studies suggested that the increased osteoclast number in the mu-calpain(-/-) bones resulted from increased osteoclast survival, not increased osteoclast formation. Calcitonin disrupted the podosome ring, induced osteoclast retraction, and reduced osteoclast motility and bone resorption in a manner similar to the effects of calpain inhibitors and had no further effect on these parameters when added to osteoclasts pretreated with calpain inhibitors. Calcitonin inhibited the constitutive cleavage of a fluorogenic calpain substrate and transiently blocked the constitutive cleavage of filamin A, talin, and Pyk2 by a protein kinase C-dependent mechanism, demonstrating that calcitonin induces the inhibition of calpain in osteoclasts. These results indicate that calpain activity is required for normal osteoclast activity and suggest that calcitonin inhibits osteoclast bone resorbing activity in part by down-regulating calpain activity.

Two Plasmodium falciparum merozoite proteins binding to erythrocyte band 3 form a direct complex

Erythrocyte invasion by malaria parasites requires multiple protein interactions. Our earlier studies showed that erythrocyte band 3 is an invasion receptor binding Plasmodium falciparum merozoite surface protein 1 and 9 (MSP1, MSP9) existing as a co-ligand complex. In this study, we have used biochemical approaches to identify the binding sites within MSP1 and MSP9 involved in the co-ligand complex formation. A major MSP9-binding site is located within the 19kDa C-terminal domain of MSP1 (MSP1(19)). Two specific regions of MSP9 defined as Delta1a and Delta2 interacted with native MSP1(19). The 42 kDa domain of MSP1 (MSP1(42)) bearing MSP1(19) in the C-terminus bound directly to both MSP9/Delta1a and Delta2. Thus, the regions of MSP1 and MSP9 interacting with the erythrocyte band 3 receptor are also responsible for assembling the co-ligand complex. Our evidence suggests a ternary complex is formed between MSP1, MSP9, and band 3 during erythrocyte invasion by P. falciparum.

Characterization of events preceding the release of malaria parasite from the host red blood cell

The process of merozoite release involves proteolysis of both the parasitophorous vacuole membrane (PVM) and red blood cell membrane (RBCM), but the precise temporal sequence remains controversial. Using immunofluorescence microscopy and Western blotting of parasite-infected RBCs, we observed that the intraerythrocytic parasite was enclosed in a continuous ring of PVM at early stages of parasite development while at the segmented schizont stage, the PVM appeared to be integrated in the cluster of newly formed merozoites. Subsequently, such clusters were detected extraerythrocytically together with single merozoites devoid of the PVM at low frequency, suggesting a primary rupture of RBCM, followed by PVM rupture and release of invasive merozoites. Secondly, since cysteine proteases are implicated in the process of parasite release, antimalarial effects of 4 cysteine protease inhibitors (leupeptin, E64, E64d, and MDL) were tested at the late schizont stage and correlated with the integrity of PVM and RBCM. We observed that leupeptin and E64 treatment produced extraerythrocytic clusters of merozoites associated with PVM suggesting inhibition of PVM lysis but not RBCM lysis. Merozoites in these clusters developed into rings upon removal of the inhibitors. In contrast, E64d and MDL caused an irreversible parasite death blocking further development. Future characterization of the mechanism(s) of inhibition may facilitate the design of novel antimalarial inhibitors.

Centaurin-α1 interacts directly with kinesin motor protein KIF13B

Centaurin-α1 is a phosphatidylinositol 3,4,5-trisphosphate binding protein as well as a GTPase activating protein (GAP) for the ADP-ribosylation factor (ARF) family of small GTPases. To further understand its cellular function, we screened a rat brain cDNA library using centaurin-α1 as bait to identify centaurin-α1 interacting proteins. The yeast two-hybrid screen identified a novel kinesin motor protein as a centaurin-α1 binding partner. The motor protein, termed KIF13B, encoded by a single ∼9.5-kb transcript, is widely expressed with high levels observed in brain and kidney. Yeast two-hybrid and GST pull-down assays showed that the interaction between centaurin-α1 and KIF13B is direct and mediated by the GAP domain of centaurin-α1 and the stalk domain of KIF13B. Centaurin-α1 and KIF13B form a complex in vivo and the KIF13B interaction appears to be specific to centaurin-α1 as other members of the ARF GAP family did not show any binding activity. We also show that KIF13B and centaurin-α1 colocalize at the leading edges of the cell periphery whereas a deletion mutant of centaurin-α1 that lacks the KIF13B binding site, failed to colocalize with KIF13B in vivo. Finally, we demonstrate that KIF13B binding suppresses the ARF6 GAP activity of centaurin-α1 in intact cells. Together, our data suggest a mechanism where direct binding between centaurin-α1 and KIF13B could concentrate centaurin-α1 at the leading edges of cells, thus modulating ARF6 function.

Human Dlg protein binds to the envelope glycoproteins of human T-cell leukemia virus type 1 and regulates envelope mediated cell-cell fusion in T lymphocytes

Human homologue of the Drosophila Dlg tumor suppressor (hDlg) is a widely expressed scaffold protein implicated in the organization of multi-protein complexes at cell adhesion sites such as the neuronal synapse. hDlg contains three PDZ domains that mediate its binding to the consensus motifs present at the C-termini of various cell surface proteins, thus inducing their clustering and/or stabilization at the plasma membrane. Using a yeast two-hybrid screen, we identified hDlg as a cellular binding partner of a viral membrane integral protein, the envelope glycoprotein (Env) of human T-cell leukemia virus type 1 (HTLV-1). HTLV-1 is a human retrovirus that infects CD4+ T lymphocytes and is preferentially transmitted via direct contacts between infected and target cells, through a structure referred to as the virological synapse. Here, we demonstrate that hDlg interacts with a classical PDZ domain-binding motif present at the C-terminus of the cytoplasmic domain of HTLV-1 Env and conserved in the related HTLV-2 virus. We further document that, in HTLV-1 infected primary T cells, hDlg and Env are concentrated in restricted areas of the plasma membrane, enriched in molecules involved in T-cell contacts. The presence of Gag proteins responsible for viral assembly and budding in these areas indicated that they constitute platforms for viral assembly and transmission. Finally, a mutant virus unable to bind hDlg exhibited a decreased ability to trigger Env mediated cell fusion between T lymphocytes. We thus propose that hDlg stabilizes HTLV-1 envelope glycoproteins at the virological synapse formed between infected and target cells, hence assisting the cell-to-cell transmission of the virus.

Association of cottontail rabbit papillomavirus E6 oncoproteins with the hDlg/SAP97 tumor suppressor

Papillomaviruses are small DNA viruses that infect epithelial tissues and cause warts. Human papillomavirus (HPV) infection is the primary risk factor for the development of cervical cancer. The E6 and E7 oncogenes are the only genes consistently expressed in HPV-positive cervical cancer cells. Cottontail rabbit papillomavirus (CRPV) induces papillomas and carcinomas on cottontail and domestic rabbits and provides an excellent animal model of HPV infection and vaccine development. CRPV encodes three transforming proteins; LE6, SE6, and E7. Each of these proteins is required for papilloma formation. Like HPV E7, the CRPV E7 protein binds to the tumor suppressor pRB. In contrast, unlike HPV E6, the CRPV E6 proteins do not bind the tumor suppressor p53. Although more than a dozen cellular proteins have been identified as HPV E6 interacting proteins, nothing is known about the cellular interacting proteins of CRPV E6s. Here we describe the association of CRPV E6s with hDlg/SAP97, the mammalian homolog of the Drosophila discs large tumor suppressor protein. HPV E6 has previously shown to bind and target hDlg/SAP97 for degradation. Our results demonstrate that both LE6 and SE6 interact with hDlg/SAP97, although their association does not lead to the degradation of hDlg/SAP97. The PDZ domains of hDlg were shown to be sufficient for interaction with CRPV E6 proteins while the C-terminus of CRPV E6 is essential for the interaction with hDlg. The association of hDlg with SE6 may be important but not sufficient for the transformation of NIH 3T3 cells by SE6. Importantly, a CRPV SE6 mutant defective for papilloma formation did not interact with hDlg. These results suggest that interaction with hDlg/SAP97 plays a role in the biological function of CRPV E6s.

Differential expression of human Dlg in cervical intraepithelial neoplasias

OBJECTIVES

To evaluate the potential role of human discs large (hDlg) protein in the pathogenesis of cervical neoplasia by examining the changes of hDlg protein expression in normal cervical epithelium as well as various stages of cervical dysplasia.

MATERIALS AND METHOD

Archived formalin-fixed, paraffin-embedded cervical tissue sections with known status of human Papillomavirus (HPV) infection were examined for hDlg expression using immunohistochemical staining by a monoclonal antibody generated against hDlg. The specimens include normal epithelium, low-grade and high-grade squamous intraepithelial lesions, and squamous cell carcinoma.

RESULTS

The hDlg protein localized primarily in the basolateral membrane of glandular columnar cells in normal endocervical epithelium. In the squamous epithelium, the hDlg staining is strong in the basal and parabasal layers and rapidly fades away in the superficial layers. Although predominantly membrane-associated, some cytoplasmic staining of hDlg is also detectable that decreases in intensity from basal to superficial layers. In low-grade squamous intraepithelial lesion, there is a moderate increase in the membranous as well as cytoplasmic staining of hDlg in the cells of superficial layer and a modest loss of membranous staining of hDlg in the basal layer. This "reverse staining pattern" for hDlg is more prominent and constant feature of high-grade squamous intraepithelial lesions. The changes of the hDlg expression are, however, invariable regardless the subtypes of HPV infection of the specimens. In the invasive squamous cell carcinoma, membranous staining of hDlg is reduced or absent with some mitotic cells showing evidence of hDlg accumulation in the midbody zone.

CONCLUSIONS

These data suggest a functional role of hDlg in the development and progression of cervical neoplasia with implications in cytokinesis, viral trafficking, and metastasis pathways.

The NMR Structure of Dematin Headpiece Reveals a Dynamic Loop That Is Conformationally Altered upon Phosphorylation at a Distal Site

Dematin (band 4.9) is found in the junctional complex of the spectrin cytoskeleton that supports the erythrocyte cell membrane. Dematin is a member of the larger class of cytoskeleton-associated proteins that contain a modular "headpiece" domain at their extreme C termini. The dematin headpiece domain provides the second F-actin-binding site required for in vitro F-actin bundling. The dematin headpiece is found in two forms in the cell, one of 68 residues (DHP) and one containing a 22-amino acid insert near its N terminus (DHP+22). In addition, dematin contains the only headpiece domain that is phosphorylated, in vivo. The 22-amino acid insert in DHP+22 appeared unstructured in NMR spectra; therefore, we have determined the three-dimensional structure of DHP by multidimensional NMR methods. Although the overall three-dimensional structure of DHP is similar to that of the villin headpiece, there are two novel characteristics revealed by this structure. First, unlike villin headpiece that contains a single buried salt bridge, DHP contains a buried charged cluster comprising residues Glu(39), Arg(66), Lys(70), and the C-terminal carboxylate of Phe(76). Second, (15)N relaxation experiments indicate that the longer "variable loop" region near the N terminus of DHP (residues 20-29) is dynamic, undergoing significantly greater motions that the rest of the structure. Furthermore, NMR chemical shift changes indicate that the conformation of the dynamic variable loop is altered by phosphorylation of serine 74, which is far in the sequence from the variable loop region. Our results suggest that phosphorylation of the dematin headpiece acts as a conformational switch within this headpiece domain.

A Co-ligand Complex Anchors Plasmodium falciparum Merozoites to the Erythrocyte Invasion Receptor Band 3

In Plasmodium falciparum malaria, erythrocyte invasion by circulating merozoites may occur via two distinct pathways involving either a sialic acid-dependent or -independent mechanism. Earlier, we identified two nonglycosylated exofacial regions of erythrocyte band 3 termed 5ABC and 6A as an important host receptor in the sialic acid-independent invasion pathway. 5ABC, a major segment of this receptor, interacts with the 42-kDa processing product of merozoite surface protein 1 (MSP1(42)) through its 19-kDa C-terminal domain. Here, we show that two regions of merozoite surface protein 9 (MSP9), also known as acidic basic repeat antigen, interact directly with 5ABC during erythrocyte invasion by P. falciparum. Native MSP9 as well as recombinant polypeptides derived from two regions of MSP9 (MSP9/Delta1 and MSP9/Delta2) interacted with both 5ABC and intact erythrocytes. Soluble 5ABC added to the assay mixture drastically diminished the binding of MSP9 to erythrocytes. Recombinant MSP9/Delta1 and MSP9/Delta2 present in the culture medium blocked P. falciparum reinvasion into erythrocytes in vitro. Native MSP9 and MSP1(42), the two ligands binding to the 5ABC receptor, existed as a stable complex. Our results establish a novel concept wherein the merozoite exploits a specific complex of co-ligands on its surface to target a single erythrocyte receptor during invasion. This new paradigm poses a new challenge in the development of a vaccine for blood stage malaria.

Protein 4.1-mediated membrane targeting of human discs large in epithelial cells

Human discs large (hDlg) protein binds to protein 4.1R via a motif encoded by an alternatively spliced exon located between the SH3 and the C-terminal guanylate kinase-like domains. To evaluate the functional significance of protein 4.1R binding for subcellular localization of hDlg in vivo, we expressed full-length recombinant constructs of two naturally occurring isoforms of hDlg termed hDlg-I2 and hDlg-I3. The hDlg-I3 but not the hDlg-I2 isoform binds to the FERM (Four.1-Ezrin-Radixin-Moesin) domain of protein 4.1R in vitro. Upon transient transfection into subconfluent Madine-Darby canine kidney (MDCK) epithelial cells, the hDlg-I3 fused with the green fluorescent protein accumulated predominantly at the plasma membrane of cell-cell contact sites, whereas the hDlg-I2 fusion protein distributed in the cytoplasm. In contrast, in stably transfected confluent MDCK cells, both hDlg-I2 and -I3 isoforms localized efficiently to the lateral membrane, consistent with the previous notion that the N-terminal domain of hDlg mediates its membrane targeting in polarized epithelial cells. We introduced a double mutation (I38A/I40A) into the N-terminal domain of hDlg, which disrupted its interaction with DLG2, a key event in the membrane targeting of hDlg. Interestingly, the hDlg-I2 isoform harboring the I38A/I40A mutation mislocalized from the membrane into cytoplasm. Importantly, the hDlg-I3 isoform with the same mutation localized efficiently to the membrane of confluent MDCK cells. Together, our results demonstrate that in addition to the N-terminal targeting domain, the alternatively spliced I3 insertion plays a critical role in recruiting hDlg to the lateral membrane in epithelial cells via its interaction with protein 4.1R.

Ankyrin peptide blocks falcipain-2-mediated malaria parasite release from red blood cells

Falcipain-2 (FP-2) is a dual-function protease that cleaves hemoglobin at the early trophozoite stage and erythrocyte membrane ankyrin and protein 4.1 at the late stages of parasite development. FP-2-mediated cleavage of ankyrin and protein 4.1 is postulated to cause membrane instability facilitating parasite release in vivo. To test this hypothesis, here we have determined the precise peptide sequence at the hydrolysis site of ankyrin to develop specific inhibitor(s) of FP-2. Mass spectrometric analysis of the hydrolysis products showed that FP-2-mediated cleavage of ankyrin occurred immediately after arginine 1,210. A 10-mer peptide (ankyrin peptide, AnkP) containing the cleavage site completely inhibited the FP-2 enzyme activity in vitro and abolished all of the known functions of FP-2. To determine the effect of this peptide on the growth and development of P. falciparum, the peptide was delivered into intact parasite-infected red blood cells (RBCs) via the Antennapedia homeoprotein internalization domain. Growth and maturation of trophozoites and schizonts was markedly inhibited in the presence of the fused AnkP peptide. <10% of new ring-stage parasites were detected compared with the control sample. Together, our results identify a specific peptide derived from the spectrin-binding domain of ankyrin that blocks late-stage malaria parasite development in RBCs. Confocal microscopy with FP-2-specific antibodies demonstrated the proximity of the enzyme in apposition with the RBC membrane, further corroborating the proposed function of FP-2 in the cleavage of RBC skeletal proteins.

Activation of transglutaminase in mu-calpain null erythrocytes

Intracellular transglutaminases (protein-glutamine: amine gamma-glutamyltransferase, EC 2.3.2.13) are calcium-dependent thiol enzymes that catalyze the covalent cross-linking of proteins, including those in the erythrocyte membrane. Several studies suggest that the activation of some transglutaminases is positively regulated by the calcium-dependent cysteine protease, mu-calpain. Using mu-calpain null (Capn1(-/-)) mouse erythrocytes, we demonstrate that the activation of soluble as well as membrane-bound forms of transglutaminase (TG2) in mouse erythrocytes was independent of mu-calpain. Also, the absence of mu-calpain or any detectable cysteine protease did not affect the transglutaminase activity in the erythrocyte lysate. Our studies also identify physiological substrates of mu-calpain in the erythrocyte membrane and show that their cleavage has no discernible effect on the transglutaminase mediated cross-linking of membrane proteins. Taken together, these data suggest the existence of a calpain-independent mechanism for the activation of transglutaminase 2 by calcium ions in the mouse erythrocytes and presumably also in non-erythroid cells.

Band 3 is a host receptor binding merozoite surface protein 1 during the Plasmodium falciparum invasion of erythrocytes

We report the molecular identification of a sialic acid-independent host-parasite interaction in the Plasmodium falciparum malaria parasite invasion of RBCs. Two nonglycosylated exofacial regions of human band 3 in the RBC membrane were identified as a crucial host receptor binding the C-terminal processing products of merozoite surface protein 1 (MSP1). Peptides derived from the receptor region of band 3 inhibited the invasion of RBCs by P. falciparum. A major segment of the band 3 receptor (5ABC) bound to native MSP1(42) and blocked the interaction of native MSP1(42) with intact RBCs in vitro. Recombinant MSP1(19) (the C-terminal domain of MSP1(42)) bound to 5ABC as well as RBCs. The binding of both native MSP1(42) and recombinant MSP1(19) was not affected by the neuraminidase treatment of RBCs, but sensitive to chymotrypsin treatment. In addition, recombinant MSP1(38) showed similar interactions with the band 3 receptor and RBCs, although the interaction was relatively weak. These findings suggest that the chymotrypsin-sensitive MSP1-band 3 interaction plays a role in a sialic acid-independent invasion pathway and reveal the function of MSP1 in the Plasmodium invasion of RBCs.

Direct interaction with a kinesin-related motor mediates transport of mammalian discs large tumor suppressor homologue in epithelial cells

Membrane-associated guanylate kinase homologues (MAGUKs) are generally found under the plasma membrane of cell-cell contact sites and function as scaffolding proteins by linking cytoskeletal and signaling molecules to transmembrane receptors. The correct targeting of MAGUKs is essential for their receptor-clustering function; however, the molecular mechanism of their intracellular transport is unknown. Here, we show that the guanylate kinase-like domain of human discs large protein binds directly within the amino acids 607-831 of the stalk domain of GAKIN, a kinesin-like protein of broad distribution. The primary structure of the binding segment, termed MAGUK binding stalk domain, is conserved in Drosophila kinesin-73 and some other motor and non-motor proteins. This stalk segment is not found in GKAP, a synaptic protein that interacts with the guanylate kinase-like domain, and unlike GKAP, the binding of GAKIN is not regulated by the intramolecular interactions within the discs large protein. The recombinant motor domain of GAKIN is an active microtubule-stimulated ATPase with k(cat) = 45 s(-1), K(0.5 (MT)) = 0.1 microm. Overexpression of green fluorescent protein-fused GAKIN in Madin-Darby canine kidney epithelial cells induced long projections with both GAKIN and endogenous discs large accumulating at the tip of these projections. Importantly, the accumulation of endogenous discs large was eliminated when a mutant GAKIN lacking its motor domain was overexpressed under similar conditions. Taken together, our results indicate that discs large is a cargo molecule of GAKIN and suggest a mechanism for intracellular trafficking of MAGUK-laden vesicles to specialized membrane sites in mammalian cells.

Plasmodium falciparum cysteine protease falcipain-2 cleaves erythrocyte membrane skeletal proteins at late stages of parasite development

Plasmodium falciparum-derived cysteine protease falcipain-2 cleaves host erythrocyte hemoglobin at acidic pH and specific components of the membrane skeleton at neutral pH. Analysis of stage-specific expression of these 2 proteolytic activities of falcipain-2 shows that hemoglobin-hydrolyzing activity is maximum in early trophozoites and declines rapidly at late stages, whereas the membrane skeletal protein hydrolyzing activity is markedly increased at the late trophozoite and schizont stages. Among the erythrocyte membrane skeletal proteins, ankyrin and protein 4.1 are cleaved by native and recombinant falcipain-2 near their C-termini. To identify the precise peptide sequence at the hydrolysis site of protein 4.1, we used a recombinant construct of protein 4.1 as substrate followed by MALDI-MS analysis of the cleaved product. We show that falcipain-2-mediated cleavage of protein 4.1 occurs immediately after lysine 437, which lies within a region of the spectrin-actin-binding domain critical for erythrocyte membrane stability. A 16-mer peptide containing the cleavage site completely inhibited the enzyme activity and blocked falcipain-2-induced fragmentation of erythrocyte ghosts. Based on these results, we propose that falcipain-2 cleaves hemoglobin in the acidic food vacuole at the early trophozoite stage, whereas it cleaves specific components of the red cell skeleton at the late trophozoite and schizont stages. It is the proteolysis of skeletal proteins that causes membrane instability, which, in turn, facilitates parasite release in vivo.

Headpiece domain of dematin is required for the stability of the erythrocyte membrane

Dematin is an actin-binding and bundling protein of the erythrocyte membrane skeleton. Dematin is localized to the spectrin-actin junctions, and its actin-bundling activity is regulated by phosphorylation of cAMP-dependent protein kinase. The carboxyl terminus of dematin is homologous to the "headpiece" domain of villin, an actin-bundling protein of the microvillus cytoskeleton. The headpiece domain contains an actin-binding site, a cAMP-kinase phosphorylation site, plays an essential role in dematin self-assembly, and bundles F-actin in vitro. By using homologous recombination in mouse embryonic stem cells, the headpiece domain of dematin was deleted to evaluate its function in vivo. Dematin headpiece null mice were viable and born at the expected Mendelian ratio. Hematological evaluation revealed evidence of compensated anemia and spherocytosis in the dematin headpiece null mice. The headpiece null erythrocytes were osmotically fragile, and ektacytometry/micropore filtration measurements demonstrated reduced deformability and filterability. In vitro membrane stability measurements indicated significantly greater membrane fragmentation of the dematin headpiece null erythrocytes. Finally, biochemical characterization, including the vesicle/cytoskeleton dissociation, spectrin self-association, and chemical crosslinking measurements, revealed a weakened membrane skeleton evidenced by reduced association of spectrin and actin to the plasma membrane. Together, these results provide evidence for the physiological significance of dematin and demonstrate a role for the headpiece domain in the maintenance of structural integrity and mechanical properties of erythrocytes in vivo.

Villin-type headpiece domains show a wide range of F-actin-binding affinities

The villin-type "headpiece" domain is a modular motif found at the extreme C-terminus of larger "core" domains in over 25 cytoskeletal proteins in plants and animals. Although headpiece is classified as an F-actin-binding domain, it has been suggested that some expressed fusion-proteins containing headpiece may lack F-actin-binding in vivo. To determine the intrinsic F-actin affinity of headpiece domains, we quantified the F-actin affinity of seven headpiece domains and three N-terminal truncations, under identical in vitro conditions. The constructs are folded and adopt the native headpiece structure. However, they show a wide range of affinities that can be grouped into high, low, and nonspecific-binding categories. Computer models of the structure and charged surface potential of these headpiece domains suggest features important for high F-actin affinity. We conclude that not all headpiece domains are intrinsically F-actin-binding motifs, and suggest that the surface charge distribution may be an important element for F-actin recognition.

Dematin interacts with the Ras-guanine nucleotide exchange factor Ras-GRF2 and modulates mitogen-activated protein kinase pathways

Erythroid dematin is a major component of red blood cell junctional complexes that link the spectrin-actin cytoskeleton to the overlying plasma membrane. Transcripts of dematin are widely distributed including human brain, heart, lung, skeletal muscle, and kidney. In vitro, dematin binds and bundles actin filaments in a phosphorylation-dependent manner. The primary structure of dematin consists of a C-terminal domain homologous to the 'headpiece' domain of villin, an actin-binding protein of the brush border cytoskeleton. Except filamentous actin, no other binding partners of dematin have been identified. To investigate the physiological function of dematin, we employed the yeast two-hybrid assay to identify dematin-interacting proteins in the adult human brain. Here, we show that dematin interacts with the guanine nucleotide exchange factor Ras-GRF2 by yeast two-hybrid assay, and this interaction is further confirmed by blot overlay, surface plasmon resonance, co-transfection, and co-immunoprecipitation assays. Human Ras-GRF2 is expressed in a variety of tissues and, similar to other guanine nucleotide exchange factors (GEFs), displays anchorage independent growth in soft agar. Co-transfection and immunoblotting experiments revealed that dematin blocks transcriptional activation of Jun by Ras-GRF2 and activates ERK1 via a Ras-GRF2 independent pathway. Because much of the present evidence has centered on the identification of the Rho family of GTPases as key regulators of the actin cytoskeleton, the direct association between dematin and Ras-GRF2 may provide an alternate mechanism for regulating the activation of Rac and Ras GTPases via the actin cytoskeleton.

Reassignment of the EPB4.1 gene to 1p36 and assessment of its involvement in neuroblastomas

OBJECTIVES

EPB4.1 has been previously mapped to human chromosome 1p33-p34.2. In contradiction to this chromosomal location, we have mapped EPB4.1-1p36 by using fluorescence in situ hybridization and radiation hybrid mapping. In neuroblastomas, deletions of the telomeric end of chromosome 1 (1p36) are the most common genetic aberration.

METHODS

We investigated whether genetic aberrations of EPB4.1 can be detected in some neuroblastomas by analyzing 72 tumours for EPB4.1 mutation, expression, and alternative splicing pattern. Furthermore, EPB4.1 protein from a neuroblastoma cell line was studied for its subcellular localization.

RESULTS

Sequence changes could be detected in 14 out of 72 neuroblastomas, including missense, silent, and intronic changes. Duplex RT-PCR analysis revealed a subset of 11 tumours expressing significantly low levels of EPB4.1. Significant EPB4.1 sequence changes that were detected included an exon 4 G/A missense mutation (amino acid: V/I) that was shown to be associated with absence of wild-type EPB4.1 expression (3 tumours), an exon 8 G/A missense mutation (V/M) (1 tumour), and an intronic sequence change that was shown to be associated with the presence of an aberrant transcript (1 tumour). Splicing pattern analysis revealed that all EPB4.1 transcripts from tumours exclude exon 3, a splicing pattern for generating the 135 kDa isoform. EPB4.1 cDNA cloned from a neuroblastoma cell line produced a 135-kDa protein with a cytoplasm/membrane localization.

CONCLUSIONS

Out of 72 neuroblastomas we have identified 11 tumours with impaired EPB4.1 expression and 5 tumours with significant sequence changes. We also found that the 135 kDa isoform is the main EPB4.1 product in neuroblastoma. EPB4.1 cDNA from a neuroblastoma cell line produced a 135-kDa protein and displayed a cytoplasm/membrane localization in transfected cells.

Schwannomin isoform-1 interacts with syntenin via PDZ domains

The neurofibromatosis type 2 gene (NF2) is involved in the pathogenesis of benign tumors of the human nervous system. The NF2 protein, called schwannomin or merlin, is inactivated in virtually all schwannomas and meningiomas. The molecular mechanisms by which schwannomin functions as a tumor suppressor is unknown but believed to involve plasma membrane-cytoskeletal interactions. Two major alternatively spliced isoforms of schwannomin differing in their C termini have been reported. Using the yeast two-hybrid system, we have identified syntenin as a binding partner for schwannomin isoform-1 (sch-1). Syntenin is an adapter protein that couples transmembrane proteoglycans to cytoskeletal components and is involved in intracellular vesicle transport. The C terminus 25 amino acids of sch-1 and the two PDZ domains of syntenin mediate their binding, and mutations introduced within the VAFFEEL region of sch-1 defined a sequence crucial for syntenin recognition. We have showed that the two proteins interacted in vitro and in vivo and localized underneath the plasma membrane. Fibroblast cells expressing heterologous antisense syntenin display alterations in the subcellular distribution of sch-1. Together, these results provide the first functional clue to the existence of schwannomin isoforms and could unravel novel pathways for the transport and subcellular localization of schwannomin in vivo.

VAM-1: a new member of the MAGUK family binds to human Veli-1 through a conserved domain

The MAGUKs (membrane-associated guanylate kinase homologues) constitute a family of peripheral membrane proteins that function in tumor suppression and receptor clustering by forming multiprotein complexes containing distinct sets of transmembrane, cytoskeletal, and cytoplasmic signaling proteins. Here, we report the characterization of the human vam-1 gene that encodes a novel member of the p55 subfamily of MAGUKs. The complete cDNA sequence of VAM-1, tissue distribution of its mRNA, genomic structure, chromosomal localization, and Veli-1 binding properties are presented. The vam-1 gene is composed of 12 exons and spans approx. 115 kb. By fluorescence in situ hybridization the vam-1 gene was localized to 7p15-21, a chromosome region frequently disrupted in some human cancers. VAM-1 mRNA was abundant in human testis, brain, and kidney with lower levels detectable in other tissues. The primary structure of VAM-1, predicted from cDNA sequencing, consists of 540 amino acids including a single PDZ domain near the N-terminus, a central SH3 domain, and a C-terminal GUK (guanylate kinase-like) domain. Sequence alignment, heterologous transfection, GST pull-down experiments, and blot overlay assays revealed a conserved domain in VAM-1 that binds to Veli-1, the human homologue of the LIN-7 adaptor protein in Caenorhabditis. LIN-7 is known to play an essential role in the basolateral localization of the LET-23 tyrosine kinase receptor, by linking the receptor to LIN-2 and LIN-10 proteins. Our results therefore suggest that VAM-1 may function by promoting the assembly of a Veli-1 containing protein complex in neuronal as well as epithelial cells.

Disruption of the mouse mu-calpain gene reveals an essential role in platelet function

Conventional calpains are ubiquitous calcium-regulated cysteine proteases that have been implicated in cytoskeletal organization, cell proliferation, apoptosis, cell motility, and hemostasis. There are two forms of conventional calpains: the mu-calpain, or calpain I, which requires micromolar calcium for half-maximal activation, and the m-calpain, or calpain II, which functions at millimolar calcium concentrations. We evaluated the functional role of the 80-kDa catalytic subunit of mu-calpain by genetic inactivation using homologous recombination in embryonic stem cells. The mu-calpain-deficient mice are viable and fertile. The complete deficiency of mu-calpain causes significant reduction in platelet aggregation and clot retraction but surprisingly the mutant mice display normal bleeding times. No detectable differences were observed in the cleavage pattern and kinetics of calpain substrates such as the beta3 subunit of alphaIIbbeta3 integrin, talin, and ABP-280 (filamin). However, mu-calpain null platelets exhibit impaired tyrosine phosphorylation of several proteins including the beta3 subunit of alphaIIbbeta3 integrin, correlating with the agonist-induced reduction in platelet aggregation. These results provide the first direct evidence that mu-calpain is essential for normal platelet function, not by affecting the cleavage of cytoskeletal proteins but by potentially regulating the state of tyrosine phosphorylation of the platelet proteins.

hCASK and hDlg associate in epithelia, and their src homology 3 and guanylate kinase domains participate in both intramolecular and intermolecular interactions

Membrane-associated guanylate kinase (MAGUK) proteins act as molecular scaffolds organizing multiprotein complexes at specialized regions of the plasma membrane. All MAGUKs contain a Src homology 3 (SH3) domain and a region homologous to yeast guanylate kinase (GUK). We showed previously that one MAGUK protein, human CASK (hCASK), is widely expressed and associated with epithelial basolateral plasma membranes. We now report that hCASK binds another MAGUK, human discs large (hDlg). Immunofluorescence microscopy demonstrates that hCASK and hDlg colocalize at basolateral membranes of epithelial cells in small and large intestine. These proteins co-precipitate from lysates of an intestinal cell line, Caco-2. The GUK domain of hCASK binds the SH3 domain of hDlg in both yeast two-hybrid and fusion protein binding assays, and it is required for interaction with hDlg in transfected HEK293 cells. In addition, the SH3 and GUK domains of each protein participate in intramolecular binding that in vitro predominates over intermolecular binding. The SH3 and GUK domains of human p55 display the same interactions in yeast two-hybrid assays as those of hCASK. Not all SH3-GUK interactions among these MAGUKs are permissible, however, implying specificity to SH3-GUK interactions in vivo. These results suggest MAGUK scaffold assembly may be regulated through effects on intramolecular SH3-GUK binding.

A cysteine protease activity from Plasmodium falciparum cleaves human erythrocyte ankyrin

The malaria parasite Plasmodium falciparum undergoes distinct morphologic changes during its 48-h life cycle inside human red blood cells. Parasite proteinases appear to play important roles at all stages of the erythrocytic cycle of human malaria. Proteases involved in erythrocyte rupture and invasion are possibly required to breakdown erythrocyte membrane skeleton. To identify such proteases, soluble cytosolic extract of isolated trophozoites/schizonts was incubated with erythrocyte membrane ghosts or spectrin-actin depleted inside-out vesicles, which were then analyzed by SDS-PAGE. In both cases, a new protein band of 155 kDa was detected. The N-terminal peptide sequencing established that the 155 kDa band represents truncated ankyrin. Immunoblot analysis using defined monoclonal antibodies confirmed that ankyrin was cleaved at the C-terminus. While the enzyme preferentially cleaved ankyrin, degradation of protein 4.1 was also observed at high concentrations of the enzyme. The optimal activity of the purified enzyme, using ankyrin as substrate, was observed at pH 7.0-7.5, and the activity was strongly inhibited by standard inhibitors of cysteine proteinases (cystatin, NEM, leupeptin, E-64 and MDL 28 170), but not by inhibitors of aspartic (pepstatin) or serine (PMSF, DFP) proteinases. Furthermore, we demonstrate that protease digestion of ankyrin substantially reduces its interaction with ankyrin-depleted membrane vesicles. Ektacytometric measurements showed a dramatic increase in the rate of fragmentation of ghosts after treatment with the protease. Although the role of ankyrin cleavage in vivo remains to be determined, based on our findings we postulate that the parasite-derived cysteine protease activity cleaves host ankyrin thus weakening the ankyrin-band 3 binding interactions and destabilizing the erythrocyte membrane skeleton, which, in turn, facilitates parasite release. Further characterization of the enzyme may lead to the development of novel antimalarial drugs.

GAKIN, a novel kinesin-like protein associates with the human homologue of the Drosophila discs large tumor suppressor in T lymphocytes

Reorganization of the cortical cytoskeleton is a hallmark of T lymphocyte activation. Upon binding to antigen presenting cells, the T cells rapidly undergo cytoskeletal re-organization thus forming a cap at the cell-cell contact site leading to receptor clustering, protein segregation, and cellular polarization. Previously, we reported cloning of the human lymphocyte homologue of the Drosophila Discs Large tumor suppressor protein (hDlg). Here we show that a novel protein termed GAKIN binds to the guanylate kinase-like domain of hDlg. Affinity protein purification, peptide sequencing, and cloning of GAKIN cDNA from Jurkat J77 lymphocytes identified GAKIN as a novel member of the kinesin superfamily of motor proteins. GAKIN mRNA is ubiquitously expressed, and the predicted amino acid sequence shares significant sequence similarity with the Drosophila kinesin-73 motor protein. GAKIN sequence contains a motor domain at the NH(2) terminus, a central stalk domain, and a putative microtubule-interacting sequence called the CAP-Gly domain at the COOH terminus. Among the MAGUK superfamily of proteins examined, GAKIN binds to the guanylate kinase-like domain of PSD-95 but not of p55. The hDlg and GAKIN are localized mainly in the cytoplasm of resting T lymphocytes, however, upon CD2 receptor cross-linking the hDlg can translocate to the lymphocyte cap. We propose that the GAKIN-hDlg interaction lays the foundation for a general paradigm of coupling MAGUKs to the microtubule-based cytoskeleton, and that this interaction may be functionally important for the intracellular trafficking of MAGUKs and associated protein complexes in vivo.

Human homologue of the Drosophila discs large tumor suppressor protein forms an oligomer in solution. Identification of the self-association site

The human homologue of the Drosophila discs large tumor suppressor protein (hDlg), a member of the membrane-associated guanylate kinase (MAGUK) superfamily, interacts with K(+) channels, N-methyl-d-aspartate receptors, calcium ATPase, adenomatous polyposis coli, and PTEN tumor suppressor proteins, and several viral oncoproteins through its PDZ domains. MAGUKs play pivotal roles in the clustering and aggregation of receptors, ion channels, and cell adhesion molecules at the synapses. To investigate the physiological basis of hDlg interactions, we examined the self-association state of full-length hDlg as well as defined segments of hDlg expressed as recombinant proteins in bacteria and insect Sf9 cells. Gel permeation chromatography of full-length hDlg revealed that the purified protein migrates as a large particle of size >440 kDa. Similar measurements of defined domains of hDlg indicated that the anomalous mobility of hDlg originated from its amino-terminal domain. Ultrastructural analysis of hDlg by low angle rotary shadow electron microscopy revealed that the full-length hDlg protein as well as its amino-terminal domain exhibits a highly flexible irregular shape. Further evaluation of the self-association state of hDlg using sedimentation equilibrium centrifugation, matrix-assisted laser desorption/ionization mass spectrometry, and chemical cross-linking techniques confirmed that the oligomerization site of hDlg is contained within its amino-terminal domain. This unique amino-terminal domain mediates multimerization of hDlg into dimeric and tetrameric species in solution. Sedimentation velocity experiments demonstrated that the oligomerization domain exists as an elongated tetramer in solution. In vitro mutagenesis was used to demonstrate that a single cysteine residue present in the oligomerization domain of hDlg is not required for its self-association. Understanding the oligomerization status of hDlg may help to explicate the mechanism of hDlg association with multimeric K(+) channels and dimeric adenomatous polyposis coli tumor suppressor protein. Our findings, therefore, begin to rationalize the role of hDlg in the clustering of membrane channels and formation of multiprotein complexes necessary for signaling and cell proliferation pathways.

Plasmodium falciparum erythrocyte membrane protein 1 is anchored to the actin-spectrin junction and knob-associated histidine-rich protein in the erythrocyte skeleton

A distinctive pathological feature of Plasmodium falciparum malaria is the endothelial attachment of erythrocytes infected with mature asexual-stage parasites in microvessels of the major organs. Electron-dense protrusions described as knobs are displayed on the surface of parasitized erythrocytes and act as attachment points in cytoadherence. Parasite-encoded knob-associated histidine-rich protein (KAHRP) is a major component of knobs found on the cytoplasmic side of the host cell membrane. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is a family of parasite-encoded cytoadherence receptors localized to knobs on the surface of parasitized erythrocytes. Despite its high antigenic diversity, PfEMP1 has a remarkably conserved cytoplasmic domain. We demonstrate in this study that the cytoplasmic domain of PfEMP1 (VAR(CD)) binds to host spectrin and actin and to full-length KAHRP in vitro. Apparent dissociation constants determined for VAR(CD)/F-actin and VAR(CD)/KAHRP interactions are 44.9+/-6.4 and 10. 7+/-2.2 nM, respectively. Further, we provide evidence that KAHRP polypeptides self-associate in solution to form structures similar to knobs and show binding of self-associated KAHRP clusters to spectrin-actin-protein 4.1 complexes. Findings in this study suggest that PfEMP1 is localized to the knob in P. falciparum-infected erythrocytes by binding to the host spectrin-actin junction and to self-associated KAHRP through its conserved cytoplasmic domain.

Loss of heterozygosity on 8p in prostate cancer implicates a role for dematin in tumor progression

Dematin is a cytoskeletal protein that bundles actin filaments in a phosphorylation-dependent manner. The primary structure of dematin is organized into an N-terminal core domain of unknown function and a C-terminal domain that is homologous to the "headpiece" domain of villin. We have previously localized the dematin gene on human chromosome 8p21.1, a region distal to the ankyrin locus for hereditary spherocytosis. Radiation hybrid mapping now places dematin between D8S258 and D8S137, two microsatellite markers frequently deleted in prostate cancer. The 8p21.1 region is also deleted in prostate, breast, colon, and bladder cancers, suggesting the presence of a tumor suppressor gene(s). Using laser-capture microdissection technique and fluorescence in situ hybridization (FISH), we demonstrate loss of heterozygosity (LOH) of the dematin gene in a majority of chromosomal region 8p21-linked prostate tumors. One allele of dematin was also deleted in the established prostate adenocarcinoma cell line PC-3, which displays a classic oncogenic phenotype. Overexpression of wild-type dematin in PC-3 cells resulted in the restoration of a more polarized, epithelial-like phenotype. Conversely, the heterologous expression of dominant negative mutants of dematin perturbed normal cell morphology of NIH 3T3 fibroblasts. These results suggest a biological function of dematin in the regulation of cell shape, with implications in the pathobiology of prostate tumorigenesis.

Exon skipping truncates the PDZ domain of human erythroid p55 in a patient with chronic myeloid leukemia in acute megakaryoblastic blast crisis

Human p55, the major palmitoylated protein associated with the cytoplasmic face of the erythrocyte membrane, is believed to modulate interactions between protein 4.1 and glycophorin C. It is the prototype of a newly described family of signaling molecules that includes hD1g, the human homologue of the Drosophila discs-large tumor suppressor protein. Chronic myeloid leukemia is characterized by transformation to a fulminating acute leukemia, heralded by evolution of the Philadelphia chromosome positive genotype (Ph +) to further abnormalities. RT-PCR of p55 mRNA from a patient with acute megakaryoblastic CML revealed a 69 base pair deletion in the PDZ domain, corresponding to exon 5 of the p55 gene. The deletion of constitutive exon 5 not only marks the first abnormality of the p55 cDNA in human disease but also the first abnormality of a PDZ domain in human disease and may represent another genetic abnormality associated with CML in blast crisis.

cDNA sequence and chromosomal localization of mouse Dlgh3 gene adjacent to the BRCA1 tumor suppressor locus

Membrane associated guanylate kinase homologues (MAGUKs) function in tumor suppression and receptor clustering pathways presumably by modulating signaling events at the interface of the membrane cytoskeleton. The p55 subclass of MAGUKs includes two novel cDNAs that were originally identified by virtue of their genomic location to human chromosome 17q12-21 where the BRCA1 tumor suppressor gene has been mapped. The predicted primary structure of the human MPP3 contains a single copy of the PDZ domain, an SH3 motif, and a carboxy-terminal guanylate kinase-like domain. Here we report the full-length coding cDNA sequence of the mouse homologue of MPP3. The translated amino acid sequence of murine Dlgh3 contains 568 amino acids that show 87% sequence identity with the human MPP3 protein. Northern blot analysis shows abundant expression of a approximately 3.0 kb transcript of Dlgh3 in mouse brain and skeletal muscle, and a relatively less abundant approximately 5.0 kb transcript in skeletal muscle, testis, kidney, and lung. Using an interspecific backcross panel, the Dlgh3 gene was mapped to a segment of mouse chromosome 11 that is conserved with human chromosome 17q12-21. The close proximity of murine Dlgh3 gene to the BRCA1 locus and the high conservation of the primary structure of human and murine proteins provide a framework for testing the role of Dlgh3 in cell proliferation pathways using the mouse as a model system.

Targeted inactivation of murine band 3 (AE1) gene produces a hypercoagulable state causing widespread thrombosis in vivo

Only 5% to 10% of band 3 null mice survive the neonatal period. To determine the cause of death, 3 adult and 11 newborn band 3 null mice were submitted for histopathologic examination. All but 1 pup showed evidence of thrombosis including: (1) large thrombotic lesions in the heart, which were partially organized, calcified in some fields, and endothelialized, indicating a process that developed premortem (3 of 3 adults and 6 of 11 pups). (2) Subcapsular necrotic areas in the liver suggestive of premortem ischemic events caused by arteriolar occlusions (8 of 11 pups). (3) Large vein thrombi (4 of 11 pups). To investigate the etiology of this hypercoagulable state, we have used the Russell's viper venom test (RVV) to show that red blood cells (RBCs) from band 3 null mice significantly shorten the RVV clotting time of normal plasma in a dose-dependent fashion, whereas RBCs from normal mice have no effect, suggesting that the membrane of band 3 null RBCs provides a suitable surface for activation of the prothrombinase complex. Using flow cytometry, we have examined the phosphatidylserine (PS)-specific binding of fluorescein isothiocyanate (FITC)-annexin V to normal and band 3 null RBCs. A subpopulation of cells (3% to 5% of RBCs) with increased FITC-annexin V binding was detected in band 3 null RBCs as compared with normal RBCs. Furthermore, the entire cell population of band 3 null RBCs shows a measurable increase in the mean fluorescence intensity, suggesting that band 3 null RBCs may have increased PS exposure on the outer membrane leaflet. These findings are further supported by direct fluorescence microscopy of normal and band 3 null RBCs labeled with FITC-annexin V. Based on these observations, we postulate that the high mortality of band 3 null mice may be related to a hypercoagulable state, which appears to originate from changes in the phospholipid composition of the membrane leading to PS exposure on the outer leaflet.

Human CASK/LIN-2 binds syndecan-2 and protein 4.1 and localizes to the basolateral membrane of epithelial cells

In Caenorhabditis elegans, mutations in the lin-2 gene inactivate the LET-23 receptor tyrosine kinase/Ras/MAP kinase pathway required for vulval cell differentiation. One function of LIN-2 is to localize LET-23 to the basal membrane domain of vulval precursor cells. LIN-2 belongs to the membrane-associated guanylate kinase family of proteins. We have cloned and characterized the human homolog of LIN-2, termed hCASK, and Northern and Western blot analyses reveal that it is ubiquitously expressed. Indirect immunofluorescence localizes CASK to distinct lateral and/or basal plasma membrane domains in different epithelial cell types. We detect in a yeast two-hybrid screen that the PDZ domain of hCASK binds to the heparan sulfate proteoglycan syndecan-2. This interaction is confirmed using in vitro binding assays and immunofluorescent colocalization. Furthermore, we demonstrate that hCASK binds the actin-binding protein 4.1. Syndecans are known to bind extracellular matrix, and to form coreceptor complexes with receptor tyrosine kinases. We speculate that CASK mediates a link between the extracellular matrix and the actin cytoskeleton via its interaction with syndecan and with protein 4.1. Like other membrane-associated guanylate kinases, its multidomain structure enables it to act as a scaffold at the membrane, potentially recruiting multiple proteins and coordinating signal transduction.

Alternative splicing and structure of the human erythroid dematin gene

Human erythroid dematin is a cytoskeletal protein capable of bundling actin filaments in vitro. The carboxyl terminal domain of dematin is homologous to the headpiece domain of villin, an actin-binding protein of the brush border cytoskeleton. Here we report the complete structure of the dematin gene located on human chromosome 8p21.1, a region frequently deleted in prostate cancer. The dematin gene is composed of 15 exons spanning approximately 15 kb. We also report two novel isoforms of dematin derived from alternative splicing of the dematin gene in the brain.

Localization of Dlg at the mammalian neuromuscular junction

Recent studies have begun to elucidate the localization of ion channels and receptors in central nervous system synapses. A family of proteins containing PDZ domains has been suggested to play essential roles in these processes. PSD-95 and chapsyn-110 have been implicated in the clustering of Shaker K+ channels and NMDA receptors in the mammalian brain, and Dlg plays a role in the clustering of Shaker K+ channels at the Drosophila neuromuscular junction (NMJ). We have explored whether Dlg might participate in mammalian NMJ organization. We demonstrate that Dlg is expressed in muscle and co-localizes with utrophin at the post-synaptic face of the mammalian NMJ. Dlg may therefore be important for establishing or maintaining the organization of protein complexes at the mammalian NMJ.

Complete deficiency of glycophorin A in red blood cells from mice with targeted inactivation of the band 3 (AE1) gene

Glycophorin A is the major transmembrane sialoglycoprotein of red blood cells. It has been shown to contribute to the expression of the MN and Wright blood group antigens, to act as a receptor for the malaria parasite Plasmodium falciparum and Sendai virus, and along with the anion transporter, band 3, may contribute to the mechanical properties of the red blood cell membrane. Several lines of evidence suggest a close interaction between glycophorin A and band 3 during their biosynthesis. Recently, we have generated mice where the band 3 expression was completely eliminated by selective inactivation of the AE1 anion exchanger gene, thus allowing us to study the effect of band 3 on the expression of red blood cell membrane proteins. In this report, we show that the band 3 -/- red blood cells contain protein 4.1, adducin, dematin, p55, and glycophorin C. In contrast, the band 3 -/- red blood cells are completely devoid of glycophorin A (GPA), as assessed by Western blot and immunocytochemistry techniques, whereas the polymerase chain reaction (PCR) confirmed the presence of GPA mRNA. Pulse-label and pulse-chase experiments show that GPA is not incorporated in the membrane and is rapidly degraded in the cytoplasm. Based on these findings and other published evidence, we propose that band 3 plays a chaperone-like role, which is necessary for the recruitment of GPA to the red blood cell plasma membrane.

Function of p55 and its nonerythroid homologues

The human erythrocyte membrane has served as a model for elucidating novel protein-protein and protein-membrane interactions that have broad implications in many nonerythroid cells. A detailed analysis of erythrocyte membrane polypeptides that migrate in the region of band 4.9 led to the cloning and characterization of p55 phosphoprotein. Subsequent studies established that the p55 protein is an obligate component of the protein 4.1-glycophorin C complex, which regulates the stability and mechanical properties of the erythrocyte plasma membrane. p55 is a member of a growing family of signaling and cytoskeletal proteins termed membrane-associated guanylate kinase homologues (MAGUKs). MAGUKs are multidomain proteins consisting of either a single or three copies of the PDZ (PSD-95/Discs large/ZO-1) domain, an SH3 motif, and a guanylate kinaselike domain. Recent studies have implicated MAGUKs in the clustering of ion channels, organization of cytoskeletal elements, cell signaling events, and regulation of cell proliferation and tumor-suppression pathways. The purpose of this review is to summarize recent developments concerning the characterization of two human MAGUKs, erythrocyte p55, and lymphocyte hDIg.