Role of Cysteines in Stabilizing the Randomized Receptor Binding Domains within Feline Leukemia Virus Envelope Proteins

Disrupting MLV integrase:BET protein interaction biases integration into quiescent chromatin and delays but does not eliminate tumor activation in a MYC/Runx2 mouse model

Murine leukemia virus (MLV) integrase (IN) lacking the C-terminal tail peptide (TP) loses its interaction with the host bromodomain and extraterminal (BET) proteins and displays decreased integration at promoter/enhancers and transcriptional start sites/CpG islands. MLV lacking the IN TP via an altered open reading frame was used to infect tumorigenesis mouse model (MYC/Runx2) animals to observe integration patterns and phenotypic effects, but viral passage resulted in the restoration of the IN TP through small deletions. Mice subsequently infected with an MLV IN lacking the TP coding sequence (TP-) showed an improved median survival by 15 days compared to wild type (WT) MLV infection. Recombination with polytropic endogenous retrovirus (ERV), Pmv20, was identified in seven mice displaying both fast and slow tumorigenesis, highlighting the strong selection within the mouse to maintain the full-length IN protein. Mapping the genomic locations of MLV in tumors from an infected mouse with no observed recombination with ERVs, TP-16, showed fewer integrations at TSS and CpG islands, compared to integrations observed in WT tumors. However, this mouse succumbed to the tumor in relatively rapid fashion (34 days). Analysis of the top copy number integrants in the TP-16 tumor revealed their proximity to known MLV common insertion site genes while maintaining the MLV IN TP- genotype. Furthermore, integration mapping in K562 cells revealed an insertion preference of MLV IN TP- within chromatin profile states associated with weakly transcribed heterochromatin with fewer integrations at histone marks associated with BET proteins (H3K4me1/2/3, and H3K27Ac). While MLV IN TP- showed a decreased overall rate of tumorigenesis compared to WT virus in the MYC/Runx2 model, MLV integration still occurred at regions associated with oncogenic driver genes independently from the influence of BET proteins, either stochastically or through trans-complementation by functional endogenous Gag-Pol protein.

Direct mass spectrometric characterization of disulfide linkages

Disulfide linkage is critical to protein folding and structural stability. The location of disulfide linkages for antibodies is routinely discovered by comparing the chromatograms of the reduced and non-reduced peptide mapping with location identification confirmed by collision-induced dissociation (CID) mass spectrometry (MS)/MS. However, CID product spectra of disulfide-linked peptides can be difficult to interpret, and provide limited information on the backbone region within the disulfide loop. Here, we applied an electron-transfer dissociation (ETD)/CID combined fragmentation method that identifies the disulfide linkage without intensive LC comparison, and yet maps the disulfide location accurately. The native protein samples were digested using trypsin for proteolysis. The method uses RapiGest SF Surfactant and obviates the need for reduction/alkylation and extensive sample manipulation. An aliquot of the digest was loaded onto a C₄ analytical column. Peptides were gradient-eluted and analyzed using a Thermo Scientific LTQ Orbitrap Elite mass spectrometer for the ETD-triggered CID MS³ experiment. Survey MS scans were followed by data-dependent scans consisting of ETD MS² scans on the most intense ion in the survey scan, followed by 5 MS³ CID scans on the 5 most intense ions in the ETD MS² scan. We were able to identify the disulfide-mediated structural variants A and A/B forms and their corresponding disulfide linkages in an immunoglobulin G2 monoclonal antibody with λ light chain (IgG2λ), where the location of cysteine linkages were unambiguously determined.