Intracellular Crosslinking of Filoviral Nucleoproteins with Xintrabodies Restricts Viral Packaging

Viruses assemble large macromolecular repeat structures that become part of the infectious particles or virions. Ribonucleocapsids (RNCs) of negative strand RNA viruses are a prime example where repetition of nucleoprotein (NP) along the genome creates a core polymeric helical scaffold that accommodates other nucleocapsid proteins including viral polymerase. The RNCs are transported through the cytosol for packaging into virions through association with viral matrix proteins at cell membranes. We hypothesized that RNC would be ideal targets for crosslinkers engineered to promote aberrant protein–protein interactions, thereby blocking their orderly transport and packaging. Previously, we had generated single-domain antibodies (sdAbs) against Filoviruses that have all targeted highly conserved C-terminal regions of NP known to be repetitively exposed along the length of the RNCs of Marburgvirus (MARV) and Ebolavirus (EBOV). Our crosslinker design consisted of dimeric sdAb expressed intracellularly, which we call Xintrabodies (X- for crosslinking). Electron microscopy of purified NP polymers incubated with purified sdAb constructs showed NP aggregation occurred in a genus-specific manner with dimeric and not monomeric sdAb. A virus-like particle (VLP) assay was used for initial evaluation where we found that dimeric sdAb inhibited NP incorporation into VP40-based VLPs whereas monomeric sdAb did not. Inhibition of NP packaging was genus specific. Confocal microscopy revealed dimeric sdAb was diffuse when expressed alone but focused on pools of NP when the two were coexpressed, while monomeric sdAb showed ambivalent partition. Infection of stable Vero cell lines expressing dimeric sdAb specific for either MARV or EBOV NP resulted in smaller plaques and reduced progeny of cognate virus relative to wild-type Vero cells. Though the impact was marginal at later time-points, the collective data suggest that viral replication can be reduced by crosslinking intracellular NP using relatively small amounts of dimeric sdAb to restrict NP packaging. The stoichiometry and ease of application of the approach would likely benefit from transitioning away from intracellular expression of crosslinking sdAb to exogenous delivery of antibody. By retuning sdAb specificity, the approach of crosslinking highly conserved regions of assembly critical proteins may well be applicable to inhibiting replication processes of a broad spectrum of viruses.

Gene cloning using cDNA libraries in a differential competition hybridization strategy: application to cloning XP-A related genes

A competition hybridization strategy using size-cut cDNA libraries as both probe and competitor was designed for the cloning of genes whose mRNAs are either regulated transcriptionally or vary in abundance as a function of cell line or cell cycle. We used this strategy to construct cDNA libraries from a particular size fraction of mRNA from three members of a xeroderma pigmentosum (XP), complementation group A family. Size-cut cDNA libraries derived from the father's, mother's, and child's fibroblast cell line were used in a competition scheme to screen two lambda gt11 human cDNA libraries. Of the 15 positive lambda gt11 clones which have been characterized, at least 14 clones represent the same gene which is present in greater abundance in both the mother and father XP obligate heterozygotes relative to the homozygous affected child.

The high molecular weight nerve growth factor complex from Mastomys natalensis differs from the murine nerve growth factor complex

Both the male and the female of Mastomys natalensis, an African rat, have high levels of nerve growth factor (NGF) in their submaxillary glands. Mastomys high molecular weight (HMW) NGF was purified by gel filtration and ion-exchange chromatography and was compared with HMW NGF from the male mouse submaxillary gland. Mastomys HMW NGF sediments as a 5S species, does not exhibit esterase activity, and is more difficult to dissociate at acid pH than mouse 7S NGF. The biological activity could be isolated as a purified Mastomys beta NGF protein identical in size and charge with that purified from male mice. The N-terminal amino acid sequence of the first 20 residues was determined and found to differ from that of mouse only at residue 8. Western blotting of Mastomys 5S NGF using antiserum against mouse beta NGF indicates that the beta NGF subunit of Mastomys is very similar to that of the mouse. Southern blots using a mouse kallikrein probe also demonstrate the presence of a large kallikrein family in Mastomys similar to that in mouse, and Northern blots verify transcription of kallikreins in Mastomys submaxillary gland. SDS-PAGE and isoelectric focusing gels reveal a Mastomys subunit that comigrates with mouse alpha subunit. However, neither oligonucleotide probes directed against mouse alpha subunit RNA nor antibodies directed against mouse alpha NGF cross-react strongly with the Mastomys material. This indicates that the second subunit of the Mastomys complex is not very similar to the mouse alpha subunit.

Apparent allosterism by avian myeloblastosis virus reverse transcriptase and E. coli DNA polymerase I

A recent report (1) presented evidence for allosterism in reverse transcription by Mason-Pfizer monkey virus reverse transcriptase and by E. coli DNA polymerase I. Our experiments also demonstrate these apparent cooperative effects when synthesis is catalyzed by either avian myeloblastosis virus DNA polymerase, feline sarcoma virus DNA polymerase, or E. coli DNA polymerase I (large fragment). We show that the apparent cooperativity depends on the use of oligo(dT)12-18 as primer. However, if the polymerase reaction products are isolated chromatographically, then the polymerases obey classical Michaelis-Menten kinetics with respect to substrate and enzyme concentrations. These results suggest that the cooperative effects are an acid precipitation artifact. The results are also consistent with the enzyme operating by a distributive mechanism with the oligo(dT)12-18 primer.