Biology subjects

Saad, J. S.

Publications and source records attributed to Saad, J. S..

3 recordsLinked to original sources

bioRxiv2021

Structural basis for human T-cell leukemia virus type 1 Gag targeting to the plasma membrane for assembly

During the late phase of retroviral replication, the virally encoded Gag polyprotein is targeted to the plasma membrane (PM) for assembly. Gag-PM binding is mediated by the N-terminal matrix (MA) domain of Gag. For many retroviruses, Gag binding to the PM was found to be dependent on phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. However, it was shown that for human T-cell leukemia virus type 1 (HTLV-1), Gag binding to membranes is less dependent on PI(4,5)P2, suggesting that other factors may modulate Gag assembly. To elucidate the mechanism by which HTLV-1 Gag binds to the PM, we employed NMR techniques to solve the structure of unmyristoylated MA (myr(-)MA) and to characterize its interactions with lipids and liposomes. The MA structure consists of four -helices and unstructured N- and C-termini. We show that myr(-)MA binds to PI(4,5)P2 via the polar head and that myr(-)MA binding to inositol phosphates (IPs) is significantly enhanced by increasing the number of phosphate groups on the inositol ring, indicating that the MA-IP binding is governed by charge-charge interactions. The IP binding site was mapped to a well-defined basic patch formed by lysine and arginine residues. Using a sensitive NMR-based liposome binding assay, we show that myr(-)MA binding to membranes is significantly enhanced by phosphatidylserine (PS). Confocal microscopy data show that Gag is localized to the inner leaflet of the PM of infected cells, while the Gag G2A mutant, lacking myristoylation, is diffuse and cytoplasmic. These findings advance our understanding of a key mechanism in retroviral assembly.

microbiology↗

bioRxiv2021

Capsule protects against intracellular killing and enables vascular endothelial cell translocation during invasive pneumococcal disease

Streptococcus pneumoniae (Spn) is a leading cause of invasive disease. Chief among its virulence determinants is capsular polysaccharide which protects the bacterium from phagocytosis. While 100 antigenically distinct capsule types are produced by Spn, i.e. serotypes, only 20-30 are commonly associated with invasive disease. A frequency that suggests serotypespecific properties of the capsule influence virulence. Herein, we show capsule has strong antioxidant properties. Moreover, that this property promotes invasive disease by protecting Spn taken up by vascular endothelial cells during bacteremia from endosome-killing and enhancing the translocation rate into organs. Crucially, isogenic capsule-switch mutants of Spn varied considerably in their resistance to H2O2-killing in culture and measured levels correlated positively with intracellular survival rates in vitro, organ invasion rates in vivo, and epidemiologically-established human attack rates for the corresponding serotype. The amount of capsule produced and specific biochemical features of a serotype, such as acetylation, also influenced Spn resistance to oxidative stress. Autolysin-mediated shedding was also found to be necessary, indicating that capsule worked as a distal sink for reactive oxygen species. Our results outline a new role for capsular polysaccharide, as an intracellular antioxidant. They help to explain why certain serotypes of Spn have greater propensity for human disease.

microbiology↗

bioRxiv2020

Structural characterization of HIV-1 matrix mutants implicated in envelope incorporation

During the late phase of HIV-1 infection, viral Gag polyproteins are targeted to the plasma membrane (PM) for assembly. Gag localization at the PM is a prerequisite for the incorporation of the envelope protein (Env) into budding particles. Gag assembly and Env incorporation are mediated by the N-terminal myristoylated matrix (MA) domain of Gag. Nonconservative mutations in the trimer interface of MA (A45E, T70R, and L75G) were found to impair Env incorporation and infectivity, leading to the hypothesis that MA trimerization is an obligatory step for Env incorporation. Conversely, Env incorporation can be rescued by a compensatory mutation in the MA trimer interface (Q63R). The impact of these MA mutations on the structure and trimerization properties of MA is not known. In this study, we employed NMR spectroscopy, x-ray crystallography, and sedimentation techniques to characterize the structure and trimerization properties of HIV-1 MA A45E, Q63R, T70R, and L75G mutant proteins. NMR data revealed that these point mutations did not alter the overall structure and folding of MA but caused minor structural perturbations in the trimer interface. Analytical ultracentrifugation data indicated that mutations had a minimal effect on the MA monomer-trimer equilibrium. The high-resolution x-ray structure of the unmyristoylated MA Q63R protein revealed hydrogen bonding between the side chains of Arg-63 and Ser-67 located in the center of the trimer interface, providing the first structural evidence for a stabilization of the trimer form. These findings advance our knowledge of the interplay of MA trimerization and Env incorporation into HIV-1 particles.

biochemistry↗