Identification of a critical basic residue on the ecotropic murine leukemia virus receptor

Targeted entry via somatostatin receptors using a novel modified retrovirus glycoprotein that delivers genes at levels comparable to those of wild-type viral glycoproteins

Here we report a novel viral glycoprotein created by replacing a natural receptor-binding sequence of the ecotropic Moloney murine leukemia virus envelope glycoprotein with the peptide ligand somatostatin. This new chimeric glycoprotein, which has been named the Sst receptor binding site (Sst-RBS), gives targeted transduction based on three criteria: (i) a gain of the use of a new entry receptor not used by any known virus; (ii) targeted entry at levels comparable to gene delivery by wild-type ecotropic Moloney murine leukemia virus and vesicular stomatitis virus (VSV) G glycoproteins; and (iii) a loss of the use of the natural ecotropic virus receptor. Retroviral vectors coated with Sst-RBS gained the ability to bind and transduce human 293 cells expressing somatostatin receptors. Their infection was specific to target somatostatin receptors, since a synthetic somatostatin peptide inhibited infection in a dose-dependent manner and the ability to transduce mouse cells bearing the natural ecotropic receptor was effectively lost. Importantly, vectors coated with the Sst-RBS glycoprotein gave targeted entry of up to 1 × 10(6) transducing U/ml, a level comparable to that seen with infection of vectors coated with the parental wild-type ecotropic Moloney murine leukemia virus glycoprotein through the ecotropic receptor and approaching that of infection of VSV G-coated vectors through the VSV receptor. To our knowledge, this is the first example of a glycoprotein that gives targeted entry of retroviral vectors at levels comparable to the natural capacity of viral envelope glycoproteins.

Turning of the receptor-binding domains opens up the murine leukaemia virus Env for membrane fusion

The activity of the membrane fusion protein Env of Moloney mouse leukaemia virus is controlled by isomerization of the disulphide that couples its transmembrane (TM) and surface (SU) subunits. We have arrested Env activation at a stage prior to isomerization by alkylating the active thiol in SU and compared the structure of isomerization-arrested Env with that of native Env. Env trimers of respective form were isolated from solubilized particles by sedimentation and their structures were reconstructed from electron microscopic images of both vitrified and negatively stained samples. We found that the protomeric unit of both trimers formed three protrusions, a top, middle and a lower one. The atomic structure of the receptor-binding domain of SU fitted into the upper protrusion. This was formed similar to a bent finger. Significantly, in native Env the tips of the fingers were directed against each other enclosing a cavity below, whereas they had turned outward in isomerization-arrested Env transforming the cavity into an open well. This might subsequently guide the fusion peptides in extended TM subunits into the target membrane.

Novel postentry resistance to AKV ecotropic mouse gammaretroviruses in the African pygmy mouse, Mus minutoides

Cells of Mus minutoides, an African pygmy mouse of the subgenus Nannomys, are susceptible to ecotropic Moloney and Friend mouse leukemia viruses (MLVs) but not to AKV-type MLVs. Transfected MA139 ferret cells expressing the mCAT-1 cell surface receptor, with the minCAT-1 substitutions K222Q and V233L, did not restrict AKV MLV. The resistance of M. minutoides cells to AKV MLV was not relieved by inhibitors of glycosylation or by the introduction of NIH 3T3 mCAT-1. Resistance is thus not mediated by receptor sequence variation, expression level, or glycosylation. M. minutoides cells are also infectible with LacZ pseudotypes having AKV Env and Moloney MLV (MoMLV) Gag proteins, further indicating that AKV Env sequence variations do not contribute to the observed block. The pattern of virus resistance in M. minutoides differs from that of the known variants of the Fv1 postentry resistance gene; M. minutoides is equally resistant to N-, B-, and NR-tropic AKV viruses and is equally susceptible to NR- and NB-tropic Friend MLVs. This novel resistance blocks replication before reverse transcription, whereas Fv1 generally restricts replication after reverse transcription; M. minutoides cells produce 2-long-terminal-repeat viral DNA circles and linear viral DNA after infection with MoMLV but not with AKV MLV. Analysis of MoMLV-AKV MLV chimeras determined that the target of resistance is in the virus capsid gene. Mutagenesis demonstrated that restriction is mediated by two amino acid substitutions, H117L and A110R; substitutions at these sites can also be targeted by the resistance genes Fv1 and TRIM5alpha. M. minutoides cells thus have a novel postentry resistance to AKV MLVs.

Ecotropic murine leukemia virus envelope protein affects interaction of cationic amino acid transporter 1 with clathrin adaptor protein complexes, leading to receptor downregulation

Mouse cationic amino acid transporter 1 (mCAT1) serves as the receptor for ecotropic murine leukemia virus (eMuLV). It has been shown that mCAT1 is expressed on the basolateral surface of polarized epithelial MDCK cells. However, little is known about the mechanisms involved in the intracellular trafficking of mCAT1. Using the green fluorescent protein-tagged mCAT1 expressed in MDCK cells, we report here that mCAT1 is physically associated with clathrin adaptor protein complex 1 (AP-1) implicated in protein trafficking from trans-Golgi network (TGN) to the basolateral surface. When the cells were infected with eMuLV, reduction of cell surface mCAT1, as well as a concomitant decrease in mCAT1-AP-1 association, was observed while association of mCAT1 with AP-3 involved in the TGN-to-lysosome trafficking was increased. Similar results were obtained when eMuLV envelope protein alone was expressed. The results may provide useful insights into the mechanism by which a simple retrovirus downregulates its receptor.

Comprehensive mapping of receptor-functioning domains in feline leukemia virus subgroup C receptor FLVCR1

Infection of cells by the highly anemogenic feline leukemia virus subgroup C (FeLV-C) is mediated by the heme exporter FLVCR1, a cell surface protein containing 12 potential transmembrane segments with six presumptive extracellular loops (ECLs). To identify FLVCR1 residues critical for mediating FeLV-C infection, we first independently isolated a human cDNA encoding the FLVCR2 protein that shares 52% identity to human FLVCR1, and we show that FLVCR2 does not function as a receptor for FeLV-C. Then, by generating specific hybrids between FLVCR1 and FLVCR2 and testing susceptibility of mouse cells expressing these hybrids to beta-galactosidase encoding FeLV-C, we identify FLVCR1 ECLs 1 and 6 as critical for mediating FeLV-C infection. Mouse cells expressing a hybrid protein containing FLVCR2 backbone with the ECL6 sequence from FLVCR1 were highly susceptible to FeLV-C infection. Using site-directed mutagenesis, we show that a single mutation of Asn463 in FLVCR2 ECL6 to an acidic Asp residue (a residue present in the corresponding position 487 in FLVCR1 ECL6) is sufficient to render FLVCR2 functional as an FeLV-C receptor. However, an Asp487Asn mutation in FLVCR1 ECL6 or substitution of the entire FLVCR1 ECL6 sequence for FLVCR2 ECL6 sequence does not disrupt receptor function. Subsequent substitutions show that residues within FLVCR1 ECL1 also contribute to mediating FeLV-C infection. Furthermore, our results suggest that FLVCR1 regions that mediate FeLV-C surface unit binding are distinct from ECL1 and ECL6. Our results are consistent with previous conclusions that infection of cells by gammaretroviruses involves interaction of virus with multiple receptor regions.

Complementation of a binding-defective retrovirus by a host cell receptor mutant

The entry of ecotropic murine leukemia virus (MLV) into cells requires the interaction of the envelope protein (Env) with its receptor, mouse cationic amino acid transporter 1 (mATRC1). An aspartic acid-to-lysine change at position 84 (D84K) of ecotropic Moloney MLV Env abolishes virus binding and infection. We recently identified lysine 234 (rK234) in mATRC1 as a residue that influences virus binding and infection. Here we show that D84K virus infection increased 3,000-fold on cells expressing receptor with an rK234A change and 100,000-fold on cells expressing an rK234D change. The stronger complementation of D84K virus infection by rK234D than by the rK234A receptor suggests that although the major reason for loss of infection of D84K and D84R virus is due to steric hindrance and charge repulsion, the loss of an interaction of D84 with receptor appears to contribute as well. Taken together, these results indicate that D84 is very close to rK234 of mATRC1 in the bound complex and there is likely an interaction between them. The definitive localization of the receptor binding site on SU should facilitate the design of chimeric envelope proteins that target infection to new receptors by replacing the receptor binding site with an exogenous ligand sequence.