Review on recent advances in the properties, production and applications of microbial dextranases

Dextranase is a type of hydrolase that is responsible for catalyzing the breakdown of high-molecular-weight dextran into low-molecular-weight polysaccharides. This process is called dextranolysis. A select group of bacteria and fungi, including yeasts and likely certain complex eukaryotes, produce dextranase enzymes as extracellular enzymes that are released into the environment. These enzymes join dextran's α-1,6 glycosidic bonds to make glucose, exodextranases, or isomalto-oligosaccharides (endodextranases). Dextranase is an enzyme that has a wide variety of applications, some of which include the sugar business, the production of human plasma replacements, the treatment of dental plaque and its protection, and the creation of human plasma replacements. Because of this, the quantity of studies carried out on worldwide has steadily increased over the course of the past couple of decades. The major focus of this study is on the most current advancements in the production, administration, and properties of microbial dextranases. This will be done throughout the entirety of the review.

Rational design of a self-assembly promoting fusion domain enhances high molecular weight levan synthesis by levansucrase SacB

The catalytic product of levansucrase from Bacillus subtilis (SacB) is mainly composed of 10 % high molecular weight levan (HMW, ~2000 kDa) and 90 % low molecular weight levan (LMW, ~7000 Da). In order to achieve efficient production of food hydrocolloid, high molecular weight levan (HMW), with the help of molecular dynamics simulation software, a protein self-assembly element, Dex-GBD, was found and fused with the C-terminus of SacB to construct a novel fusion enzyme, SacB-GBD. The product distribution of SacB-GBD was reversed compared with SacB, and the proportion of HMW in the total polysaccharide was significantly increased to >95 %. We then confirmed that the self-assembly was responsible for the reversal of the SacB-GBD product distribution by the simultaneous modulation of SacB-GBD particle size and product distribution by SDS. The hydrophobic effect may be the main driver of self-assembly as analyzed by molecular simulations and hydrophobicity determination. Our study provides an enzyme source for the industrial production of HMW and provides a new theoretical basis for guiding the molecular modification of levansucrase towards the size of the catalytic product.

Dysregulation of the endomembrane homeostasis triggers vascular smooth muscle cell phenotype switching

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Deutsche Forschungsgemeinschaft

Background

Cardiovascular diseases are a leading cause of mortality globally, with cardiovascular calcification as a prominent predictor and contributor. Notably, microcalcifications might cause atherosclerotic plaque rupture, leading to myocardial infarction. Calcified plaque remodeling accompanies phenotypic modulation of vascular smooth muscle cells (SMCs). Previously, we showed that extracellular vesicles (EVs) released from calcifying SMCs are pivotal for microcalcification formation. EVs originate from the endolysosomal system. The lipid kinase FYVE-type zinc finger containing phosphoinositide kinase (PIKfyve) plays a role in the endolysosomal maturation. Here, we investigated how modulation of the endolysosomal system intersects with SMC phenotype regulation.

Methods and Results

In calcified arteries and SMCs, we observed increased PIKfyve expression. Inhibition of PIKfyve with the small molecule Apilimod inhibited osteogenic differentiation of SMCs in an in vitro calcification model. Apilimod reduced tissue non-specific alkaline phosphatase (TNAP) – an early marker of SMC calcification on mRNA (-79%, p<0.01), protein (-96%, p<0.01) and activity (-92%, p<0.01) levels. Additionally, matrix mineralization and collagen network formation are inhibited. SMC-secreted EVs exhibited a lower mineral content and reduced aggregation potential assessed by osteosense-based flow cytometry and turbidity assays. Transcriptome analyses revealed that the anti-calcifying effect is accompanied by a decreased expression of contractile SMC genes. Validation by RT-PCR supported that SMC-specific markers (ACTA2, SM22) and the key SMC transcription factor MYOCD were downregulated, while the expression of macrophage-like SMC markers (LGALS3, CD68) was induced. On the other hand, Apilimod increased the expression of adipogenic transcription factors (CEBPA, PPARG) and genes of the cholesterol and fatty acid metabolism pathways (SREBF2, HMGCR, FABP3, CD36) and induced the nuclear translocation of PPARγ, the main regulator of adipogenic differentiation. Fluorescence-based assays revealed increased fatty acid and disrupted low-density lipoprotein (LDL) uptake in Apilimod-treated SMCs. In Ldlr-deficient mice fed a high-fat, high-cholesterol diet for 15 weeks and treated with Apilimod for 5 weeks, we found differentially distributed plaque lipid depositions detected by Oil Red O staining.

Conclusion

Disrupting the endolysosomal maturation by PIKfyve inhibition ameliorates SMC calcification and induces a phenotypic adipogenic-like, pro-inflammatory adaption. Our data elucidates the importance of SMC phenotype monitoring during therapeutic interference. The exact lineage characteristics of these SMCs remain to be further investigated.

The effect of the neutral cytidine protonated analogue pseudoisocytidine on the stability of i-motif structures

Incorporation of pseudoisocytidine (psC), a neutral analogue of protonated cytidine, in i-motifs has been studied by spectroscopic methods. Our results show that neutral psC:C base pairs can stabilize i-motifs at neutral pH, but the stabilization only occurs when psC:C base pairs are located at the ends of intercalated C:C⁺ stacks. When psC occupies central positions, the resulting i-motifs are only observed at low pH and psC:C⁺ or psC:psC⁺ hemiprotonated base pairs are formed instead of their neutral analogs. Overall, our results suggest that positively charged base pairs are necessary to stabilize this non-canonical DNA structure.

Multiorgan engraftment of human somatic cells in swine foetuses after intra-blastocyst transplantation

Adult human stem cells, mainly from hematopoietic lineage, have been injected into developing pre-immune animal foetuses, and xenogenic engraftment of liver and other organs has been reported. We isolated a rare cell population from adult human liver, fat and skin. Colonies with few cells became visible as early as 2-3 days, and a fully formed colony took 10-14 days to form. These colonies were named as liver-derived cell lines (LDCs), fat-derived cell lines (FDCs) and skin-derived cell lines (SDCs). All these cells express few pluripotency markers like Klf4, c-myc and Sox2. Pig blastocysts were injected with LDCs, FDCs and SDCs and transferred to recipient pigs. We achieved an overall pregnancy rate of 71.4% at day 35. The foetuses were analysed for human cell chimerism in liver, kidney and heart both by RT-PCR and real-time PCR using primers specific to human and pig mitochondrial DNA. The percentage of foetuses showing chimerism was 17.4% (4/23), 12.5% (2/16) and 11.1% (1/9) for LDCs, FDCs and SDCs, respectively. Of these, 42.9% (three out of seven) showed chimerism in liver and 71.4% (five out of seven) showed kidney chimerism. However, we did not detect any chimerism in the heart. The level of chimerism varied and was in the range of one human cell per one hundred thousand to one million pig cells.

231 SELECTION OF UBIQUITOUS AND ENDOTHELIUM-SPECIFIC PROMOTERS FOR EXPRESSING TRANSGENES IN PIG

Several promoters of different origins have been used to drive expression of transgenes in pig with variable results. However, there has not been a systemic search to identify the most reliable, ubiquitous, and tissue-specific promoters, such as endothelium-specific promoters, to drive expression of transgenes with high predictability. In this study we tested 9 ubiquitous mammalian promoters, generated by assembling enhancers, minimal promoters, and 5′untranslated region of different origins (Table 1) and 5 endothelial-specific promoters: hFlt-1 (fms-like tyrosine kinase), hICAM-2 (intercellular adhesion molecule-2), andhEndoglin promoters from human; mouse Tie-2 promoter (Schlaeger et al. 1997 PNAS 94, 3058-3063); and pig ICAM-2 promoter (Godwin et al. 2006 Xenotransplantation 13, 514-521). These promoters drive the expression of either GFP or LacZ reporter genes and were electroporated into 9 different pig cell types: endothelium-derived 2A2 and PEDSV15 cells (Seebach et al. 2001 Xenotransplantation 8, 48-61), nasal fibroblasts, kidney epithelial cells, and lung macrophages (ATCC, CRL-2528, 2842, and 2843), pig fetal fibroblasts, liver stem cells, fat-derived stem cells, and immortalized liver endothelial cells (generated in the lab). The expression was graded as week, moderate, or strong depending on the brightness of GFP signal or intensity of LacZ staining. Out of 9 ubiquitous promoters, 7 showed either low expression in all cell types or different levels of expression in some of the cell types. However, 2 promoters, hCMV-hCMV-HTLV and SV40-hCMV-HTLV, showed strong and stable expression across all 9 cell types. Among endothelial-specific promoters, pig ICAM-2 promoter showed moderate and specific expression in all 3 endothelial cell types (strong expression with the enhancer) and no expression in non-endothelial cells. Human EC-specific promoter hFlt-1 showed moderate expression in all 3 endothelial cell types, as well as in some of the non-endothelial cells such as liver stem cells, fat-derived stem cells, and lung macrophages. The other endothelial-specific human and mouse promoters did not show any expression in both endothelial and non-endothelial cells. In summary, we identified hCMV-hCMV-HTLV and SV40-hCMV-HTLV as 2 ubiquitous promoters and pICAM2 as an endothelial-specific promoter for expression of transgenes in pig. In the future, we will test these 3 promoters for in vivo expression studies. Table 1.Ubiquitous promoter characteristics

435 REDUCTION OF CELL CYCLE LENGTH AND INCREASE IN S-PHASE BY GROWTH FACTORS IN PIG FETAL FIBROBLASTS

Gene targeting in primary somatic cells is inefficient compared with embryonic stem cells. This is because of a slow rate of cell proliferation, fewer cells in S-phase at a given time point under normal culture conditions, and low rate of homologous recombination. Homologous recombination occurs mainly in late S-phase and increase in gene targeting efficiency has been reported in S-phase synchronized cells in bovine and rhesus macaque fetal fibroblasts. In this study we tested several growth factors: platelet-derived growth factor (PDGF), tumor necrosis factor a (TNFα), epidermal growth factor (EGF), fibroblast growth factor (FGF), transforming growth factor β1 (TGFβ1), insulin-like growth factor 1 (ILGF-1) and insulin-like growth factor II (ILGF-II) individually and in various combinations to see the effect on cell proliferation rate. Each experimental set consisted of 3 replicates. TGFβ1-, ILGF1-, ILGFII-, and FGF-treated cells grew very slowly compared with untreated cells. However, a combination of 3 growth factors: PDGF (15 ng mL-1), EGF (50 ng mL-1) and TNFa (100 pg mL-1), herein referred to as the cocktail, accelerated cell proliferation rate and reduced cell cycle length on average from 24.5 ± 0.2 to 20.4 ± 0.5 h with no significant change in number of cells in S-phase. Further, cells grown in the presence of the cocktail showed changes in morphology. The cells became spindle-shaped and occupied less surface area per cell compared with untreated cells. Importantly, cocktail-treated cells maintained a normal karyotype without any chromosomal abnormality. Thymidine has been used successfully to block various cell types in S-phase but it failed to synchronize these cells in S-phase in the concentration range of 2 to 10 mM for 24 to 48 h. However, serum starvation (0.2% fetal bovine serum) for 48 h blocked the cell proliferation rate effectively and synchronized cells in G0 phase (80-82% cells). After releasing from the block, cells were grown in the absence or presence of cocktail and cell cycle analysis was done at different time points by flow cytometry. Each time point was repeated 3 times. We observed the maximum number of cells in S-phase at 22 to 23 h (61.33% ± 7.77 in cocktail-treated cells v. 41.7% ± 3.28 in untreated cells). In summary, the cocktail-treated cells showed changes in cell morphology, higher proliferation rate, reduction in cell cycle length by 16.7%, and maximum percentage of cells in S-phase following serum starvation but maintained normal karyotypes. This high proliferation rate, reduction in cell cycle length, and maximum number of cells in S-phase should be very helpful in increasing the efficiency of gene-targeting in pig fetal fibroblasts.

66 ISOLATION, CHARACTERIZATION, AND NUCLEAR REPROGRAMMING OF STEM CELLS DERIVED FROM ADULT PORCINE LIVER AND FAT

Genetic manipulation of the porcine genome to produce genetically modified pigs for various biomedical and agriculture applications has been hampered by the lack of an ideal cell type that can grow in culture for a long period of time and is amenable to various genetic manipulations with high efficiency. The cell type currently used for various genetic manipulations is fetal fibroblast. These cells have very limited life span in culture and the efficiency of gene targeting is very low. There is no report of isolation of functional embryonic stem cells (ESC) from pig that would have been used to produce transgenic pigs with high efficiency as has been possible in mice. Recently, porcine induced pluripotent stem cells (iPSC) have been reported by 3 groups. However, they have yet to be tested for genetic manipulations and production of transgenic pigs. In this study, we developed a simple but novel strategy to recover stem cells from adult porcine liver and adipose tissues. Small colonies with few cells became visible as early as 2 to 3 days under reduced oxygen conditions on collagen-coated plates, and a full-grown colony with a fibroblast-like morphology took 10 to 14 days to form. Ten single colonies per tissue were isolated, subcultured, and monitored for growth and gene expression. Both of these cell types maintained steady growth through 70 population doublings (at the time of writing) and are still growing without any change in their morphology. Reverse transcription PCR was used to monitor gene expression. Both cell types show strong expression of c-Myc and KLF4. Moreover, low expression of Oct-4 and Lin28, 2 important genes related to pluripotency, was also detected in cells derived from adipose tissues, indicating that these cells could be easily reprogrammed to pluripotent state. These cells maintained a normal karyotype after long-term culture. Cell lines with stable genetic modifications and extended expression of transgene were obtained when these cells were transfected with a plasmid containing the neomycin resistance gene and selected under G418. Further, these 2 cell types, liver stem cells (LSC) and fat stem cells (FSC), and fetal fibroblasts (FF) as a control were used as nuclear donors to produce somatic cell nuclear transfer (SCNT) embryos. The average fusion rates were 87, 81, and 89% for LSC, FSC, and FF, respectively. Of 2 recipients receiving nuclear transfer embryos produced with each cell type, one established pregnancy at Day 30 (50%). Efficiencies were 5% (11 fetuses/223 embryos transferred), 1.8% (4 fetuses/228 embryos transferred),and 5% (11 fetuses/219 embryos transferred) for LSC, FSC, and FF, respectively. Thus, these adult liver and fat stem cells are attractive cell types for cloning valuable adult animals with high efficiency and for SCNT transgenesis.

263 USE OF PORCINE PARTHENOTES AND GENE EXPRESSION PROFILING USING MICROARRAYS FOR IDENTIFICATION OF IMPRINTED GENES

Genomic imprinting arises from differential epigenetic markings including DNA methylation and histone modifications and results in one allele being expressed in a parent-of-origin specific manner. For further insight into the porcine epigenome, gene expression profiles of parthenogenetic (PRT; two maternally derived chromosome sets) and biparental embryos (BP; one maternal and one paternal set of chromosomes) were compared using microarrays. Comparison of the expression profiles of the two tissue types permits identification of both maternally and paternally imprinted genes and thus the degree of conservation of imprinted genes between swine and other mammalian species. Diploid porcine parthenogenetic fetuses were generated using follicular oocytes (BOMED, Madison, WI, USA). Oocytes with a visible polar body were activated using a single square pulse of direct current of 50 V/mm for 100 �s and diploidized by culture in 10 �g/mL cycloheximide for 6 h to limit extrusion of the second polar body. Following culture, BP embryos obtained by natural matings, and PRT embryos, were surgically transferred to oviducts on the first day of estrus. Fetuses recovered at 28-30 days of gestation were dissected to separate viscera including brain, liver, and placenta; the visceral tissues were then flash-frozen in liquid nitrogen. Porcine fibroblast tissue was obtained from the remaining carcass by mincing, trypsinization, and plating cells in �-MEM. Total RNA was extracted from frozen tissue or cell culture using RNA Aqueous kit (Ambion, Austin, TX, USA) according to the manufacturer's protocol. Gene expression differences between BP and PRT tissues were determined using the GeneChip� Porcine Genome Array (Affymetrix, Santa Clara, CA) containing 23 256 transcripts from Sus scrofa and representing 42 genes known to be imprinted in human and/or mice. Triplicate arrays were utilized for each tissue type, and for PRT versus BP combination. Significant differential gene expression was identified by a linear mixed model analysis using SAS 5.0 (SAS Institute, Cary, NC, USA). Storey's q-value method was used to correct for multiple testing at q d 0.05. The following genes were classified as imprinted on the basis of their expression profiles: In fibroblasts, ARHI, HTR2A, MEST, NDN, NNAT, PEG3, PLAGL1, PEG10, SGCE, SNRPN, and UBE3A; in liver, IGF2, PEG3, PLAGL1, PEG10, and SNRPN; in placenta, HTR2A, IGF2, MEST, NDN, NNAT, PEG3, PLAGL1, PEG10, and SNRPN; and in brain, none. Additionally, several genes not known to be imprinted in humans/mice were highly differentially expressed between the two tissue types. Overall, utilizing the PRT models and gene expression profiles, we have identified thirteen genes where imprinting is conserved between swine and humans/mice, and several candidate genes that represent potentially imprinted genes. Presently, our efforts are focused in the identification of single nucleotide polymorphisms (SNPs) to more carefully evaluate the behavior of these genes in normal and abnormal gestations and to test whether the candidate genes are indeed imprinted. This research was supported by USDA-CSREES grant 524383 to J. P. and B. F.

51 PHENOTYPIC VARIATION IN CLONED SWINE IS CORRECTED IN THE F1 GENERATION

Systematic studies of cloned animals generated from adult somatic cell nuclei are critical in assessing the utility of somatic cell cloning in various applications, including the safety of food products from cloned animals and their offspring. Studies in mice show that abnormalities seen in the cloned parents are not transmitted to the siblings. To our knowledge, however, there are no studies on the F1 progeny of clones from food animals. Previously, we compared somatic cell derived cloned pigs with naturally bred control pigs on a series of physiological and genetic parameters. Phenotypic and genetic analyses indicated that there are two classes of traits, one in which the cloned pigs have less variation than controls and another characterized by variation that is equally high in cloned and control pigs. We have extended our studies to the F1 progeny of these clones to see whether these phenotypic differences are transmitted to the next generation. Age-, sex-, and breed-matched cloned and control pigs, housed together since weaning, were used in this study. Starting with their second estrus cycle, standing gilts were mated two consecutive days. All gilts were mated to the same boar. Pregnant cloned (n = 9) and naturally bred (n = 5) gilts (F0) were allowed to farrow naturally, and number and sex of live offspring at birth (F1) recorded. There was no difference in the average litter size between litters from cloned gilts and naturally bred controls (7.78 ± 2.6 and 7.40 ± 3.0, respectively; mean ± SD) or in the degree of litter size variation (coefficients of variation of 33.4% and 40.5% for litters of clones and controls, respectively). Similarly there were no statistical differences between sex ratios from cloned litters (51%:49%; M:F) and control litters (59%:41%; M:F). Blood profiles among cloned pigs, control pigs, and their progeny were compared at two time points, i.e. 15 and 27 weeks, to quantify the effect of cloning on various blood parameters and their transmission to next generation. Although the range of values for all traits overlapped between different classes, the variation differed between F0 clones and F0 controls. In the clones there were two groups of traits: one in which cloned pigs had less variability than controls, and the other in which clones had the same variability as control pigs. In contrast, the variability between all of the traits in F1 progeny of both the clones and the control pigs was similar at 15 and 27 weeks, with one exception. Combined, our data and previous results in mice strongly support the hypothesis that offspring of clones are to all intent and purposes indistinguishable from offspring of naturally bred animals, and as such there should not be any increased risks associated with consumption of products from these animals. This work was supported from NIH grant HL 51587 to JAP.

343NUCLEAR LOCALIZATION SIGNAL AND CELL SYNCHRONY ENHANCES GENE TARGETING EFFICIENCY IN FETAL BOVINE FIBROBLASTS

The use of primary somatic cells for nuclear transfer has facilitated the manipulation of the domestic animal genome via homologous recombination. Yet, the absolute frequency of homologous recombination (HR) in somatic cells is two orders of magnitude lower than in ES cells whereas frequencies of non-homologous end joining are higher. While a few loci have been targeted in somatic cells using enrichment strategies similar to those used in mouse ES cells, there have been problems of low efficiency, mixed targeted and non-targeted cells, and difficulties in cloning the cell after targeting. We present evidence that the use of a nuclear localization signal (nls) and S-phase cell cycle synchronization by thymidine block enhances targeting efficiency at the hypoxanthine phosphoribosyl transferase locus in primary fetal bovine fibroblasts. We designed two hypoxanthine phosphoribosyl transferase (HPRT)-targeting constructs, HPRT-DEx6 and HPRT-DEx6-nls. Both constructs have a 31-bp deletion and a PGK-puro insertion in exon 6 to ensure inactivation of the HPRT locus. Additionally, the HPRT-DEx6-nls construct contains a 180-bp cassette comprised of two 72-bp tandem repeats from SV40 enhancer known to act as nuclear localization signal. Diploid male cells that undergo targeted gene disruption at the single copy X-linked HPRT locus can be selected with 8-Azaguanine (8-AG) as HPRT cells are resistant to 8-AG; all transformants, random and targeted, can be selected in puromycin. Male primary bovine fibroblasts were electroporated with linearized targeting constructs, and plated in media containing puromycin or puromycin plus 8AG. All experiments were done in triplicate and data were analyzed by two-way ANOVA with NLS and cell synchrony as the two factors. Significance was set at P<0.01. While the total number of insertions (random plus targeted) with both constructs were equivalent, the HPRT-DEx6-nls construct produced a significantly higher number of targeted colonies (1–2 8AG-resistant colonies per 9.5×106 cells) than HPRT-DEx6 where no targeted events were seen. Cells were synchronized in the S-phase of the cell cycle by a 2mM thymidine treatment for 24 hours and electroporated with the linearized targeting constructs. Compared to non-synchronous cells, the total number of insertions (random plus targeted) was reduced by 59-fold in constructs with or without nls (P<0.01), while targeted insertions increased 6-fold in the HPRT-DEx6-nls construct, from an average of 10 per million cells without nls to 7.6 per 10 million with nls (P<0.01). All 8AG-resistant colonies were verified by Long Range-PCR, and PCR products confirmed by end sequencing. This finding has important implications for targeting in somatic cells, as with a drastic reduction in the number of random insertions, and increased targeting due to the presence of the nls, identification of a targeted colony is greatly facilitated even in cases where no enrichment protocols are available.

A comparative evaluation of the effects of oral lorazepam, alprazolam and diazepam on venous admixture

OBJECTIVE

To compare the effects of oral diazepam, lorazepam and alprazolam premedication on venous admixture.

MATERIAL AND METHODS

One hundred fifty patients divided in three groups were included in the study. The venous admixture was determined using the ISO-shunt nomogram. The values obtained 90 minutes after administration of the drugs were compared with the values before the drug administration. The Student's t-test was applied to find out the significance.

RESULTS

These were highly significant change in increase in venous admixture (Qs/Qt) in group I patients 90 minutes after premedication as compared to premedication values. There was statistically insignificant difference in venous admixture (Qs/Qt) in group II and group III patients 90 minutes after premedication as compared to premedication values.

CONCLUSION

From the present study it can be concluded that 2 mg of oral lorozepam given 90 minutes before surgery to healthy patients have significant effects on venous admixture. However, the effects of alprazolam and diazepam had no significant effect on venous admixture.

A genetic and mosaic analysis of a locus involved in the anesthesia response of Drosophila melanogaster

We describe a genetic and behavioral analysis of several alleles of har38, a mutant with altered sensitivity to the general anesthetic halothane. We obtained a P-element-induced allele of har38 and generated several excision alleles by remobilizing the P element. The mutants narrow abdomen (na) and har85 are confirmed to be allelic to har 38. Besides a decreased sensitivity to halothane, all mutant alleles of this locus cause a characteristic walking behavior in the absence of anesthetics. We have quantified this behavior using a geotaxis apparatus. Responses of the mutant alleles to different inhalational anesthetics were tested. The results strongly favor a multipathway model for the onset of anesthesia. Mosaic flies were tested for their response to halothane and checked for their abnormal walking behavior. The analysis suggests that both the behaviors are exhibited only by such mosaics as have the entire head of mutant origin. It is likely that this focus represents an element of a common pathway in the anesthetic response to several inhalational anesthetics but not all. This result is the first demonstration of regional specificity in the CNS of any animal for general anesthetic action.

Basic amino acid residue cluster within nuclear targeting sequence motif is essential for cytoplasmic plectin-vimentin network junctions

We have generated a series of plectin deletion and mutagenized cDNA constructs to dissect the functional sequences that mediate plectin's interaction with intermediate filament (IF) networks, and scored their ability to coalign or disrupt intermediate filaments when ectopically expressed in rat kangaroo PtK2 cells. We show that a stretch of approximately 50 amino acid residues within plectin's carboxy-terminal repeat 5 domain serves as a unique binding site for both vimentin and cytokeratin IF networks of PtK2 cells. Part of the IF-binding domain was found to constitute a functional nuclear localization signal (NLS) motif, as demonstrated by nuclear import of cytoplasmic proteins linked to this sequence. Site directed mutagenesis revealed a specific cluster of four basic amino acid residues (arg4277-arg4280) residing within the NLS sequence motif to be essential for IF binding. When mutant proteins corresponding to those expressed in PtK2 cells were expressed in bacteria and purified proteins subjected to a sensitive quantitative overlay binding assay using Eu3+-labeled vimentin, the relative binding capacities of mutant proteins measured were fully consistent with the mutant's phenotypes observed in living cells. Using recombinant proteins we also show by negative staining and rotary shadowing electron microscopy that in vitro assembled vimentin intermediate filaments become packed into dense aggregates upon incubation with plectin repeat 5 domain, in contrast to repeat 4 domain or a mutated repeat 5 domain.