Cytoplasmic domain of protocadherin‐α enhances homophilic interactions and recognizes cytoskeletal elements

Cell adhesion molecules of the protocadherin‐α (pcdh‐α), ‐β, and ‐γ families have been proposed to be synaptic specifiers. Pcdh‐α and ‐γ family members localize in part to synapses, and deletion of all pcdh‐γs in mouse affects synaptogenesis. Little is known, however, about the binding specificities and intracellular signaling of protocadherins. Using heterologous expression of tagged constructs, immunostaining, and biotinylation of surface components followed by Western blots we demonstrate that pcdh‐αs undergo homophilic interactions that are significantly enhanced by the cytoplasmic domain. Pcdh‐αs cloned from chick ciliary ganglion have one of two cytoplasmic constant regions (A‐ and B‐types). Screening a yeast two‐hybrid library of ciliary ganglion cDNA with the A‐type domain yielded a fragment of neurofilament M (NFM); screening with B‐type domain yielded a fragment of the actin‐bundling protein fascin. Cotransfection of HEK cells with the constructs indicated that the NFM and A‐type fragments codistributed as did the fascin and B‐type fragments, and the latter could be coimmunoprecipitated. Antibody‐induced clustering of full‐length pcdh‐αs on the surface of transfected HEK cells induced coclustering of the interacting NFM fragment. Native full‐length NFM in tissue extracts bound specifically to the A‐type domain on beads, while native full‐length fascin in tissue extracts specifically coimmunoprecipitated with pcdh‐α. Immunostaining neurons demonstrated codistribution of full‐length pcdh‐α with both NFM and actin filaments. These findings suggest cytoskeletal links for pcdh‐αs and identify candidate targets. They also demonstrate homophilic interactions for pcdh‐αs as described for classical cadherins. © 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

Characterization of the Head-to-Tail Overlap Complexes Formed by Human Lamin A, B1 and B2 “Half-minilamin” Dimers

Half-minilamins, representing amino- and carboxy-terminal fragments of human lamins A, B1 and B2 with a truncated central rod domain, were investigated for their ability to form distinct head-to-tail-type dimer complexes. This mode of interaction represents an essential step in the longitudinal assembly reaction exhibited by full-length lamin dimers. As determined by analytical ultracentrifugation, the amino-terminal fragments were soluble under low ionic strength conditions sedimenting with distinct profiles and s-values (1.6-1.8 S) indicating the formation of coiled-coil dimers. The smaller carboxy-terminal fragments were, except for lamin B2, largely insoluble under these conditions. However, after equimolar amounts of homotypic amino- and carboxy-terminal lamin fragments had been mixed in 4 M urea, upon subsequent renaturation the carboxy-terminal fragments were completely rescued from precipitation and distinct soluble complexes with higher s-values (2.3-2.7 S) were obtained. From this behavior, we conclude that the amino- and carboxy-terminal coiled-coil dimers interact to form distinct oligomers (i.e. tetramers). Furthermore, a corresponding interaction occurred also between heterotypic pairs of A- and B-type lamin fragments. Hence, A-type lamin dimers may interact with B-type lamin dimers head-to-tail to yield linear polymers. These findings indicate that a lamin dimer principally has the freedom for a “combinatorial” head-to-tail association with all types of lamins, a property that might be of significant importance for the assembly of the nuclear lamina. Furthermore, we suggest that the head-to-tail interaction of the rod end domains represents a principal step in the assembly of cytoplasmic intermediate filament proteins too.     

Adenylyl cyclases from Plasmodium, Paramecium and Tetrahymena are novel ion channel/enzyme fusion proteins

In Paramecium, cAMP formation is stimulated by a potassium conductance, which is an intrinsic property of the adenylyl cyclase. We cloned a full-length cDNA and several gDNA fragments from Paramecium and Tetrahymena coding for adenylyl cyclases with a novel domain composition. A putative N-terminal ion channel domain contains a canonical S4 voltage-sensor and a canonical potassium pore-loop located C-terminally after the last transmembrane span on the cytoplasmic side. The adenylyl cyclase catalyst is C-terminally located. DNA microinjection of a green fluorescent protein (GFP)-tagged construct into the macronucleus of Paramecium resulted in ciliary localization of the expressed protein. An identical gene coding for an ion-channel adenylyl cyclase was cloned from the malaria parasite Plasmodium falciparum. Expression of the catalytic domain of the latter in Sf9 cells yielded an active homodimeric adenylyl cyclase. The occurrence of this highly unique subtype of adenylyl cyclase appears to be restricted to ciliates and apicomplexa.     

Teneurin 2 is expressed by the neurons of the thalamofugal visual system in situ and promotes homophilic cell-cell adhesion in vitro

The transmembrane glycoprotein teneurin 2 is expressed by neurons in the developing avian thalamofugal visual system at periods that correspond with target recognition and synaptogenesis. Partial and full-length teneurin 2 constructs were expressed in cell lines in vitro. Expression of the cytoplasmic domain is required for the induction of filopodia, the transport of teneurin 2 into neurites and the co-localization of teneurin 2 with the cortical actin cytoskeleton. In addition, expression of the extracellular domain of teneurin 2 by HT1080 cells induced cell aggregation, and the extracellular domain of teneurin 2 became concentrated at sites of cell-cell contact in neuroblastoma cells. These observations indicate that the homophilic binding of teneurin 2 may play a role in the development of specific neuronal circuits in the developing visual system.     

Intermediate Filament Disassembly in Cultured Dorsal Root Ganglion Neurons Is Associated with Amino-Terminal Head Domain Phosphorylation of Specific Subunits

Abstract: We previously reported that activation of protein kinase A in cultured rat dorsal root ganglion neurons, treated concomitantly with low concentrations of okadaic acid that selectively inhibit protein phosphatase-2A, enhanced the Triton X-100 solubility of neurofilament triplet proteins. We now show that peripherin and α-internexin follow the same fragmentation profile as the neurofilament subunits, consistent with the notion that all five cytoplasmic intermediate filament proteins in these neurons form an integrated filamentous network whose assembly can be modulated by protein kinase A. Similar to the situation previously observed for the light neurofilament subunit, there was a strong correlation between phosphorylation of the amino-terminal head domain of peripherin and filament fragmentation. In contrast, insignificant levels of ³²P were incorporated into α-internexin under conditions promoting disassembly, indicating that phosphorylation of this protein is not involved directly in filament fragmentation. The situation for the mid-sized neurofilament subunit (NFM) was not as clear-cut. Phosphopeptide mapping of NFM revealed many head and tail domain phosphorylation sites. However, changes in NFM head domain phosphorylation under conditions promoting filament disassembly were not as pronounced as for peripherin.     

Partial cleavage of A-type lamins concurs with their total disintegration from the nuclear lamina during apoptosis

Although activated caspase 6 is capable of cleaving both A- and B-type lamins during apoptosis, the higher-order structure of the nuclear lamina may cause a differential breakdown of these two types of lamins. In order to obtain a better understanding of the dynamics and the consequences of the rapid, coordinated breakdown of the lamina complex, we applied the green fluorescent protein (GFP) technology in living cells, in which the fate of individual caspase cleavage fragments of A- and B-type lamins was examined. CHO-K1 cells were stably transfected with cDNA constructs encoding N-terminally GFP-labelled hybrids of lamin A, lamin Adelta10, lamin C or lamin B1. The course of the apoptotic process, induced by the kinase inhibitor staurosporine or by the proteasome inhibitor MG132, was monitored by digital imaging microscopy or confocal microscopy. Time-lapse recordings showed that parallel to DNA condensation N-terminally GFP-tagged A-type lamins became diffusely dispersed throughout the nucleoplasm and rapidly translocated to the cytoplasm. In contrast, the majority of GFP-lamin B1 signal remained localised at the nuclear periphery, even after extensive DNA condensation. Comparison of lamin B1-GFP signal with A-type lamin antibody staining in the same apoptotic cells confirmed the temporal differences between A- and B-type lamina dispersal. Immunoblotting revealed only a partial cleavage of A-type lamins and an almost complete cleavage of lamin B1 during apoptosis. In contrast to lamin B1 in normal cells, this cleaved lamin B1, which is apparently still associated with the nuclear membrane, can be completely extracted by methanol or ethanol. Fluorescence loss of intensity after photobleaching experiments showed that in apoptotic cells A-type lamin-GFP molecules diffuse almost freely in both nucleoplasm and cytoplasm, while the lamin B1-GFP fragments remain more stably associated with the nuclear membrane, which is confirmed by co-localisation immunofluorescence studies with a nucleoporin p62 antibody. Our results therefore clearly show a differential behaviour of A- and B-type lamins during apoptosis, suggesting not only distinct differences in the organisation of the lamina filaments, but also that caspase cleavage of only a small fraction of A-type lamins is needed for its complete disintegration.     

The focal adhesion kinase amino-terminal domain localises to nuclei and intercellular junctions in HEK 293 and MDCK cells independently of tyrosine 397 and the carboxy-terminal domain

The function and intracellular localisation of the non-catalytic NH(2)-terminal region of focal adhesion kinase (FAK) are unclear. We investigated the targetting of the FAK NH(2)-terminal domain in HEK 293 and epithelial MDCK cells. Exogenous expression of a variety of GFP-fused and epitope-tagged NH(2) terminal domain constructs either including or lacking the major Tyr 397 autophosphorylation and Src-binding site targeted to nuclei and cell-cell junctions in HEK 293 cells and co-localised at junctions with occludin, and beta1 integrin subunits at junctions. Mutation of Tyr 397 also had no effect on localisation of the NH(2)-terminal domain. In contrast, constructs encoding either the kinase or focal adhesion targeting (FAT) domains but lacking the NH(2)-terminal region failed to localise to intercellular junctions or nuclei. The NH(2)-terminal domain was not associated with beta1 integrin subunits as indicated by co-immunoprecipitation experiments, but did co-localise with cortical actin filaments. The NH(2)-terminal domain also targetted to nuclei and intercellular junctions in MDCK cells, whereas full-length FAK localised only to focal adhesions in these cells. These results indicate that the FAK NH(2)-terminal domain targets to epithelial intercellular junctions and nuclei and suggest novel functions for FAK NH(2)-terminal domain fragments independent of Y397, kinase, and FAT domains.     

Head and/or CaaX domain deletions of lamin proteins disrupt preformed lamin A and C but not lamin B structure in mammalian cells

The nuclear lamina is an important determinant of nuclear architecture. Mutations in A-type but not B-type lamins cause a range of human genetic disorders, including muscular dystrophy. Dominant mutations in nuclear lamin proteins have been shown to disrupt a preformed lamina structure in Xenopus egg extracts. Here, a series of deletion mutations in lamins A and B1 were evaluated for their ability to disrupt lamina structure in Chinese hamster ovary cells. Deletions of either the lamin A "head" domain or the C-terminal CaaX domain formed intranuclear aggregates and resulted in the disruption of endogenous lamins A/C but not lamins B1/B2. By contrast, "head-less" lamin B1 localized to the nuclear rim with no detectable effect on endogenous lamins, whereas lamin B1 CaaX domain deletions formed intranuclear aggregates, disrupting endogenous lamins A/C but not lamins B1/B2. Filter binding assays revealed that a head/CaaX domain lamin B1 mutant interacted much more strongly with lamins A/C than with lamins B1/B2. Regulated induction of this mutant in stable cell lines resulted in the rapid elimination of all detectable lamin A protein, whereas lamin C was trapped in a soluble form within the intranuclear aggregates. In contrast to results in Xenopus egg extracts, dominant negative lamin B1 (but not lamin A) mutants trapped replication proteins involved in both the initiation and elongation phases of replication but did not effect cellular growth rates or the assembly of active replication centers. We conclude that elimination of the CaaX domain in lamin B1 and elimination of either the CaaX or head domain in lamin A constitute dominant mutations that can disrupt A-type but not B-type lamins, highlighting important differences in the way that A- and B-type lamins are integrated into the lamina.     

Topography of horizontal cells in the retina of the domestic cat.

Neurofibrillar methods stain a class of horizontal cells in the cat retina which are shown to be identical with the A-type horizontal cell of Golgi-staining. Thus all of the A-type cells of a single retina can be observed. On this basis the changes in density and dendritic field size of A-type horizontal cells with respect to retinal eccentricity were measured. The decrease in density from centre to periphery is balanced by a corresponding increase in size of the dendritic field. Consequently each retinal point--independent of retinal position--is covered by the dendritic fields of three of four A-type horizontal cells. The nuclei and nucleoli of B-type horizontal cells could also be recognized in neurofibrillar-stained material and thus their distribution was determined. The density ratio B-type: A-type is 2.8 +/- 0.4 and does not vary much from the centre to the periphery of the retina. Each retinal point is also covered by four B-type horizontal cells. Thus a single cone can contact a maximum of eight horizontal cells. The rate of density decrease from centre to periphery is closely similar in cones and horizontal cells but greater in ganglion cells.     

Reconstitution of a native-like SH2 domain from disordered peptide fragments examined by multidimensional heteronuclear NMR

The N-terminal SH2 domain from the p85alpha subunit of phosphatidylinositol 3' kinase is cleaved specifically into 9- and 5-kD fragments by limited proteolytic digestion with trypsin. The noncovalent SH2 domain complex and its constituent tryptic peptides have been investigated using high-resolution heteronuclear magnetic resonance (NMR). These studies have established the viability of the SH2 domain as a fragment complementation system. The individual peptide fragments are predominantly unstructured in solution. In contrast, the noncovalent 9-kD + 5-kD complex shows a native-like (1)H-(15)N HSQC spectrum, demonstrating that the two fragments fold into a native-like structure on binding. Chemical shift analysis of the noncovalent complex compared to the native SH2 domain reveals that the highest degree of perturbation in the structure occurs at the cleavage site within a flexible loop and along the hydrophobic interface between the two peptide fragments. Mapping of these chemical shift changes on the structure of the domain reveals changes consistent with the reduction in affinity for the target peptide ligand observed in the noncovalent complex relative to the intact protein. The 5-kD fragment of the homologous Src protein is incapable of structurally complementing the p85 9-kD fragment, either in complex formation or in the context of the full-length protein. These high-resolution structural studies of the SH2 domain fragment complementation features establish the suitability of the system for further protein-folding and design studies.     

beta-Catenin associates with the actin-bundling protein fascin in a noncadherin complex

Catenins were first characterized as linking the cytoplasmic domains of cadherin cell-cell adhesion molecules to the cortical actin cytoskeleton. In addition to their essential role in modulating cadherin adhesivity, catenins have more recently been indicated to participate in cell and developmental signaling pathways. beta-Catenin, for example, associates directly with at least two receptor tyrosine kinases and transduces developmental signals within the Wnt pathway. Catenins also complex with the tumor suppressor protein adenomatous polyposis coli (APC), which appears to have a role in regulating cell proliferation. We have used the yeast two-hybrid method to reveal that fascin, a bundler of actin filaments, binds to beta-catenin's central Armadillo repeat domain. Western blotting of immunoprecipitates from cell line and mouse and rat brain extracts indicate that this interaction exists in vivo. Fascin and beta-catenin's association was further substantiated in vitro using purified proteins isolated from recombinant bacterial and baculoviral sources. Immunoprecipitation analysis indicates that fascin additionally binds to plakoglobin, which is highly homologous to beta-catenin but not to p120cas, a newly described catenin which contains a more divergent Armadillo-repeat domain. Immunoprecipitation, in vitro competition, and domain-mapping experiments demonstrate that fascin and E-cadherin utilize a similar binding site within beta-catenin, such that they form mutually exclusive complexes with beta-catenin. Immunofluorescence microscopy reveals that fascin and beta-catenin colocalize at cell-cell borders and dynamic cell leading edges of epithelial and endothelial cells. In addition to cell-cell borders, cadherins were unexpectedly observed to colocalize with fascin and beta-catenin at cell leading edges. It is conceivable that beta-catenin participates in modulating cytoskeletal dynamics in association with the microfilament-bundling protein fascin, perhaps in a coordinate manner with its functions in cadherin and APC complexes.     

Lco1 is a novel widely expressed lamin-binding protein in the nuclear interior

A-type lamins are localized at the nuclear envelope and in the nucleoplasm, and are implicated in human diseases called laminopathies. In a yeast two-hybrid screen with lamin C, we identified a novel widely expressed 171-kDa protein that we named Lamin companion 1 (Lco1). Three independent biochemical assays showed direct binding of Lco1 to the C-terminal tail of A-type lamins with an affinity of 700 nM. Lco1 also bound the lamin B1 tail with lower affinity (2 microM). Ectopic Lco1 was found primarily in the nucleoplasm and colocalized with endogenous intranuclear A-type lamins in HeLa cells. Overexpression of prelamin A caused redistribution of ectopic Lco1 to the nuclear rim together with ectopic lamin A, confirming association of Lco1 with lamin A in vivo. Whereas the major C-terminal lamin-binding fragment of Lco1 was cytoplasmic, the N-terminal Lco1 fragment localized in the nucleoplasm upon expression in cells. Furthermore, full-length Lco1 was nuclear in cells lacking A-type lamins, showing that A-type lamins are not required for nuclear targeting of Lco1. We conclude that Lco1 is a novel intranuclear lamin-binding protein. We hypothesize that Lco1 is involved in organizing the internal lamin network and potentially relevant as a laminopathy disease gene or modifier.     

The cytoplasmic domain of the myelin P0 protein influences the adhesive interactions of its extracellular domain

The extracellular domain of the myelin P0 protein is believed to engage in adhesive interactions and thus hold the myelin membrane compact. We have previously shown that P0 can behave as a homophilic adhesion molecule through interactions of its extracellular domains (Filbin, M. T., F. S. Walsh, B. D. Trapp, J. A. Pizzey, and G. I. Tennekoon. 1990. Nature (Lond.) 344:871-872). To determine if the cytoplasmic domain of P0 must be intact for the extracellular domains to adhere, we compared the adhesive capabilities of P0 proteins truncated at the COOH-terminal to the full-length P0 protein. P0 cDNAs lacking nucleotides coding for the last 52 or 59 amino acids were transfected into CHO cells, and surface expression of the truncated proteins was assessed by immunofluorescence, surface labeling followed by immunoprecipitation, and an ELISA. Cell lines were chosen that expressed at least equivalent amounts of the truncated P0 proteins at the surface as did a cell line expressing the full-length P0. The adhesive properties of these three cell lines were compared. It was found that when a suspension of single cells was allowed to aggregate for a period of 60 min, only the cells expressing the full-length P0 had formed large aggregates, while the cells expressing the truncated P0 molecules were still mostly single cells indistinguishable from the control cells. Furthermore, 25-30% of the full-length P0 was insoluble in NP40, indicative of an interaction with the cytoskeleton, whereas only 5-10% of P0 lacking 52 amino acids and none of P0 lacking 59 amino acids were insoluble. These results suggest that for the extracellular domain of P0 to behave as a homophilic adhesion molecule, its cytoplasmic domain must be intact, and most probably, it is interacting with the cytoskeleton.     

Constitutively signaling fragments of Tsr, the Escherichia coli serine chemoreceptor.

Tsr, the serine chemoreceptor of Escherichia coli, has two signaling modes. One augments clockwise (CW) flagellar rotation, and the other augments counterclockwise (CCW) rotation. To identify the portion of the Tsr molecule responsible for these activities, we isolated soluble fragments of the Tsr cytoplasmic domain that could alter the flagellar rotation patterns of unstimulated wild-type cells. Residues 290 to 470 from wild-type Tsr generated a CW signal, whereas the same fragment with a single amino acid replacement (alanine 413 to valine) produced a CCW signal. The soluble components of the chemotaxis phosphorelay system needed for expression of these Tsr fragment signals were identified by epistasis analysis. Like full-length receptors, the fragments appeared to generate signals through interactions with the CheA autokinase and the CheW coupling factor. CheA was required for both signaling activities, whereas CheW was needed only for CW signaling. Purified Tsr fragments were also examined for effects on CheA autophosphorylation activity in vitro. Consistent with the in vivo findings, the CW fragment stimulated CheA, whereas the CCW fragment inhibited CheA. CheW was required for stimulation but not for inhibition. These findings demonstrate that a 180-residue segment of the Tsr cytoplasmic domain can produce two active signals. The CCW signal involves a direct contact between the receptor and the CheA kinase, whereas the CW signal requires participation of CheW as well. The correlation between the in vitro effects of Tsr signaling fragments on CheA activity and their in vivo behavioral effects lends convincing support to the phosphorelay model of chemotactic signaling.     

Effects of preganglionic denervation and postganglionic axotomy on acetylcholine receptors in the chick ciliary ganglion

The regulation of nicotinic acetylcholine receptors (AChRs) in chick ciliary ganglia was examined by using a radiolabeled anti-AChR mAb to quantitate the amount of receptor in ganglion detergent extracts after preganglionic denervation or postganglionic axotomy. Surgical transection of the preganglionic input to the ciliary ganglion in newly hatched chicks caused a threefold reduction in the total number of AChRs within 10 d compared with that present in unoperated contralateral control ganglia. Surgical transection of both the choroid and ciliary nerves emerging from the ciliary ganglion in newly hatched chicks to establish postganglionic axotomy led to a nearly 10-fold reduction in AChRs within 5 d compared with unoperated contralateral ganglia. The declines were specific since they could not be accounted for by changes in ganglionic protein or by decreases in neuronal survival or size. Light microscopy revealed no gross morphological differences between neurons in operated and control ganglia. A second membrane component of cholinergic relevance on chick ciliary ganglion neurons is the alpha-bungarotoxin (alpha-Bgt)-binding component. The alpha-Bgt-binding component also declined in number after either postganglionic axotomy or preganglionic denervation, but appeared to do so with a more rapid time course than did ganglionic AChRs. The results imply that cell-cell interactions in vivo specifically regulate both the number of AChRs and the number of alpha-Bgt-binding components in the ganglion. Regulation of these neuronal cholinergic membrane components clearly differs from that previously described for muscle AChRs.     

Proteolytic processing of endogenous and recombinant beta 4 integrin subunit

The alpha 6 beta 4 integrin is a receptor involved in the interaction of epithelial cells with basement membranes. This integrin is unique among the known integrins in that its beta 4 subunit has a large cytoplasmic domain. The function of this cytoplasmic domain is not known. In this paper we show that the beta 4 subunit undergoes proteolytic processing in cultured cells and provide evidence that this also happens in tissues. Immunoprecipitation experiments indicated that the cytoplasmic domain of beta 4 is susceptible to a calcium-dependent protease present in cellular extracts. In vitro assays with purified calpain showed that this enzyme can cleave beta 4 at two distinct sites in the cytoplasmic domain, generating truncated molecules of 165 and 130 kD. Immunoblotting experiments performed on cultured epithelial cells using an antibody to a peptide modeled after the COOH-terminus of the beta 4 subunit showed 70-kD fragments and several fragments of molecular masses between 185 and 115 kD. Similar fragments were detected in CHO cells transfected with the full-length beta 4 cDNA, but not in control transfected cells or in cells transfected with a mutant cDNA lacking the epitope of the cytoplasmic peptide antibody. The sizes of the fragments indicated that both the intracellular and extracellular domains of beta 4 are proteolytically processed. To examine the processing of the beta 4 subunit in epithelial tissues in vivo, human skin frozen sections were stained with antibodies to the ectodomain or the cytoplasmic domain of beta 4. The distinct staining patterns obtained with the two types of antibodies provided evidence that beta 4 is proteolytically processed in vivo in skin. Analogous experiments performed on sections of the cornea suggested that beta 4 is not proteolytically processed at a detectable level in this tissue. Thus, cleavage of the beta 4 subunit occurs in a tissue-specific fashion. These results suggest a potential mechanism of modulating the activities of the alpha 6 beta 4 integrin.     

Lipid-binding role of betaII-spectrin ankyrin-binding domain

It is known that erythroid and non-erythroid spectrins binding of vesicles and monolayers containing PE proved sensitive to inhibition by red blood cell ankyrin. We now show that the bacterially-expressed recombinant peptides representing betaII(brain)-spectrin's ankyrin-binding domain and its truncated mutants showed lipid-binding activity, although only those containing a full-length amino terminal fragment showed high to moderate affinity towards phospholipid mono- and bilayers and a substantial sensitivity of this binding to inhibition by ankyrin. These results are in accordance with our published data on betaI-spectrin's ankyrin-binding domain [Hryniewicz-Jankowska A, et al. Mapping of ankyrin-sensitive, PE/PC mono- and bilayer binding site in erythroid beta-spectrin. Biochem J 2004;382:677-85]. Moreover, we tested also the effect of transient transfection of living cells of several cell-lines with vectors coding for GFP-conjugates including betaII and also betaI full-length ankyrin-binding domain and their truncated fragments on the membrane skeleton organization. The transfection with constructs encoding full-length ankyrin-binding domain of betaII and betaI spectrin resulted in increased aggregation of membrane skeleton and its punctate appearance in contrast to near normal appearance of membrane skeleton of cells transiently transfected with GFP control or construct encoding ankyrin-binding domain truncated at their N-terminal region. Our results therefore indicate the importance of N-terminal region for lipid-binding activity of the beta-spectrin ankyrin-binding domain and its substantial role in maintaining the spectrin-based skeleton distribution.     

Insights into the oligomeric states, conformational changes, and helicase activities of SV40 large tumor antigen

The large T (LT) antigen encoded by SV40 virus is a multi-domain, multi-functional protein that can not only transform cells but can also function as an efficient molecular machine to unwind duplex DNA for DNA replication. Here we report our findings on the oligomeric forms, domain interactions, and ATPase and helicase activities of various LT constructs. For the LT constructs that hexamerize, only two oligomeric forms, hexameric and monomeric, were detected in the absence of ATP/ADP. However, the presence of ATP/ADP stabilizes LT in the hexameric form. The LT constructs lacking the N- and C-terminal domains, but still retaining hexamerization ability, have ATPase as well as helicase activities at a level comparable to the full-length LT, suggesting the importance of hexamerization for these activities. The domain structures and the possible interactions between different LT fragments were probed with limited protease (trypsin) digestion. Such protease digestion generated a distinct pattern in the presence and absence of ATP/ADP and Mg(2+). The most C-terminal fragment (residues 628-708, containing the host-range domain), which was thought to be completely unstructured, was somewhat trypsin-resistant despite the presence of multiple Arg and Lys, possibly due to a rather structured C terminus. Furthermore, the N- and C-terminal fragments cleaved by trypsin were associated with other parts of the molecule, suggesting the interdomain interactions for the fragments at both ends.