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Table 1. Retrovirus interference groups in human cells

Interference group

Virus

Description

Human chromosome that encodes receptor

1

RD114

Cat endogenous virus

19

 

SNV

Avian spleen necrosis virus

 

 

BaEV

Baboon endogenous virus

 

 

SRV-1

Simian retrovirus

 

 

SRV-2

Simian retrovirus

 

 

SRV-3 (MPMV)

Simian retrovirus

 

 

SRV-4

Simian retrovirus

 

 

SRV-5

Simian retrovirus

 

 

PO-1-Lou

Spectacled langur retrovirus

 

 

SMRV

Squirrel monkey retrovirus

 

2

MLV-A

Amphotropic murine leukemia virus

8

3

MLV-X

Xenotropic murine leukemia virus

 

4

FeLV-C

Feline leukemia virus

 

5

FeLV-B

Feline leukemia virus

2

 

SSAV

Simian sarcoma-associated virus

 

 

GALV

Gibbon ape leukemia virus

 

6

BLV

Bovine leukemia virus

 

7

HTLV-1

Human T-cell leukemia virus

17

 

HTLV-2

Human T-cell leukemia virus

 

 

ChTLV

Chimpanzee T-cell leukemia virus

 

 

STLV

Simian T-cell leukemia virus

 

8

HIV-1

Human immunodeficiency virus

12

 

HIV-2

Human immunodeficiency virus

 

 

SIV

Simian immunodeficiency virus

 

Interference data are from Sommerfelt and Weiss (5), and for SNV, from Kewalramani et al. (6). Chromosome localization data are from the following references: group 1 (7), group 2 (8), group 5 (9), group 7 (10), and group 8 (11).

more recently, a second protein related to fusin and previously named CC-CKR-5 has been found to be a coreceptor for macrophage-tropic HIV-1 strains (15, 16).

These results, showing that two proteins are required for HIV-1 entry, raise the possibility that coreceptors are required for entry of other retroviruses. However, their detection will require the identification of nonpermissive cells for which transfer of the known receptors does not render the cells susceptible to infection. Some retroviruses have a very wide host range; thus, if other proteins are required for entry of these viruses, functional homologs of these coreceptors must be widely distributed in cells from many species.

Two of the cloned retrovirus receptors, Ram1 and Glvr1, are closely related at the protein sequence level (21, 22, 24), and both are sodium-dependent phosphate transporters (23). These proteins are members of a large family of known and presumptive phosphate transporters from many organisms (Fig. 1). However, Ram1 and Glvr1 are clearly distinct since the genes encoding these proteins are located on different chromosomes in humans and mice (8, 9, 30, 31) and they show very different patterns of expression in animal tissues (23). In addition, these proteins serve as receptors for distinct groups of viruses in human cells (Table 1).

The 10A1 Retrovirus Can Use Either of Two Receptors for Cell Entry

Studies of cloned retrovirus receptors and most virus interference data suggested that individual retroviruses bind to a single protein for entry into cells. When different viruses bind to the same receptor, they typically show reciprocal interference; that is, infection of cells by either virus blocks entry by the other virus. The finding of nonreciprocal interference between some retroviruses complicated this picture. In the example shown (Table 3), transduction by a vector with an amphotropic, a 10A1, or an ecotropic pseudotype was measured in NIH 3T3 mouse cells infected with amphotropic MLV (AM-MLV), 10A1 MLV, Moloney MLV, or no virus. A typical pattern of interference for viruses that use different receptors for cell entry is shown by the amphotropic and ecotropic viruses, where ecotropic vector transduction is blocked by the presence of ecotropic MoMLV in the target cells, but is unaffected by the presence of amphotropic virus, and ampho-

Table 2. Cloned retrovirus receptors

Retrovirus

Receptor

Type*

Function

Refs.

Human immunodeficiency virus

CD4

TM1

Immune recognition

12, 13

 

Fusin, CC-CKR-5 (coreceptors)

TM7

G protein-coupled chemokine receptors

1416

Simian immunodeficiency virus

CD4

TM1

Immune recognition

17

Murine ecotropic retrovirus

Rec1

TM14

Basic amino acid transport

1820

Murine amphotropic retrovirus

Ram1

TM10–13

Phosphate transport

2123

Gibbon ape leukemia virus

Glvr1

TM10–13

Phosphate transport

23, 24

Bovine leukemia virus

Blvr

TM1

ND

25, 26

Avian leukosis virus type A

Tva

TM1

LDL receptor-like protein

27

Feline immunodeficiency virus

CD9

TM4

Signaling protein?

28, 29

ND, not determined; LDL, low density lipoprotein.

*TM followed by a number indicates the number of transmembrane domains in the protein.



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