Characterization of the Proline-Rich Region of Murine Leukemia Virus Envelope Protein

Abstract

Mammalian type C retroviral envelope proteins contain a variable proline-rich region (PRR), located between the N-terminal receptor-binding domain and the more highly conserved C-terminal portion of the surface (SU) subunit. We have investigated the role of the PRR in the function of murine leukemia virus (MuLV) envelope protein. In the MuLVs, the PRR contains a highly conserved N-terminal sequence and a hypervariable C-terminal sequence. Despite this variability, the amphotropic PRR could functionally substitute for the ecotropic PRR. The hypervariable region of the PRR was not absolutely required for envelope protein function. However, truncations in this region resulted in decreased levels of both the SU and TM proteins in viral particles and increased amounts of the uncleaved precursor protein, Pr85. In contrast, the N-terminal conserved region was essential for viral infectivity. Deletion of this region prevented the stable incorporation of envelope proteins into viral particles in spite of normal envelope protein processing, wild-type levels of cell surface expression, and a wild-type ability to induce syncytia in an XC cell cocultivation assay. However, higher levels of the SU protein were shed into the supernatant, suggesting a defect in SU-TM interactions. Our data are most consistent with a role for the PRR in stabilizing the overall structure of the protein, thereby affecting the proper processing of Pr85, SU-TM interactions, and the stable incorporation of envelope proteins into viral particles. In addition, we have demonstrated that the PRR can tolerate the insertion of a peptide-binding domain, making this a potentially useful site for constructing targetable retroviral vectors.

Cell cycle checkpoint control: The cyclin G1/Mdm2/p53 axis emerges as a strategic target for broad‑spectrum cancer gene therapy - A review of molecular mechanisms for oncologists

“…While the gristly pathobiology of the tumor extracellular matrix (ECM), as well as the molecular mechanisms of retroviral vector-mediated gene transfer and expression, are beyond the scope of this focused review, the potential clinical implications of an ‘active’ tumor-targeted gene delivery vehicle are wide-ranging. Focusing on the abnormal collagenous proteins (i.e., lesion signature proteins) that are pathologically exposed during the process of tissue injury (such as balloon angioplasty), Hall and Gordon adaptively engineered a physiological surveillance function embodied within the complex structure of von Willebrand coagulation Factor (vWF), which normally guides platelets to sites of significant tissue injury, into the surface envelope protein (gp70) of the Moloney murine leukemia virus, thereby creating a desirable ‘gain-of-function’ (pathological-matrix-targeting), without impairing the natural receptor-mediated binding and entry of the targeted viral particles into target cells, thus preserving the efficiency of gene transfer ( 79 , 80 ). Eventually, in anticipation of human gene therapy applications, the enabling tumor-targeting gain-of-function, first established in rodents, was genetically engineered into the 4070A ‘amphotropic’ murine leukemia virus envelope protein that is capable of transducing human cells ( 81 , 82 ).…”