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Journal of Virology, March 2000, p. 2926-2929, Vol. 74, No. 6
Department of Molecular and Structural
Biology1 and Department of Medical
Microbiology and Immunology,2 University of
Aarhus, DK-8000 Aarhus C, Denmark
Received 23 July 1999/Accepted 8 December 1999
Feline leukemia virus subgroup B (FeLV-B) and gibbon ape leukemia
virus (GALV) utilize the human protein Pit1 but not the related
protein, Pit2, as receptor. A stretch of 9 amino acids, named region A,
was identified in the putative fourth extracellular loop of Pit1
(residues 550 through 558) as critical for FeLV-B and GALV receptor
function. However, the presence of Pit1 region A did not confer
receptor function for FeLV-B upon Pit2, while it did so for GALV. We
have here shown that the presence of two Pit1-specific loop 4 residues
(tyrosine 546 and valine 548) in addition to Pit1 region A is
sufficient to make Pit2 an efficient FeLV-B receptor; that is, a
stretch of 13 amino acids encompassing all loop 4 amino acid
differences between Pit1 and Pit2 comprises a C-terminal determinant
for FeLV-B receptor function. Thus, the same limited receptor region is
sufficient to confer receptor function for both viruses upon Pit2.
Feline leukemia virus subgroup
B (FeLV-B), Gibbon ape leukemia virus (GALV),
Amphotropic murine leukemia virus (A-MuLV), and 10A1
MuLV all belong to the genus of mammalian type C retroviruses. Their
cellular receptors have been cloned from various species including
humans, hamsters, cats, rats, and mice (7, 8, 16, 17, 19, 24, 29,
30, 32, 33). The human receptor for FeLV-B and GALV is Pit1
(formerly GLVR1) (19, 28), and the human receptor for A-MuLV
is the related protein (62% amino acid identity) Pit2 (formerly GLVR2)
(30). Pit1 is not an efficient A-MuLV receptor, and
similarly, Pit2 does not support FeLV-B or GALV entry, while 10A1 can
efficiently use both Pit1 and Pit2 as receptors (17, 22).
Based on hydropathy plots and the observation that the Pit genes seem
to have arisen by a gene duplication, Pit1 and Pit2 have been predicted
to be multipass transmembrane proteins with 10 transmembrane regions, 5 extracellular loops, and a large intracellular hydrophilic domain
(3, 7, 30). Their cellular function is sodium-dependent
phosphate transport (9, 20, 31).
Within the putative fourth extracellular loop, a stretch of 9 amino
acids (Pit1 positions 550 through 558), named region A, shows
considerable amino acid variation among Pit1, Pit2, and their homologs
from other species. Several studies have shown that amino acid residues
in region A are critical for both FeLV-B and GALV receptor function
(5, 8, 17, 22, 23, 25-27). For GALV, the presence of human
Pit1 region A conferred receptor function upon MusPit1, while it was
abolished in the reciprocal chimera (7, 8). These chimeras
exhibited the same receptor pattern for FeLV-B as observed for GALV,
demonstrating a similar critical role of region A in FeLV-B entry
(27). Moreover, substitution of Pit1 region A for that of
Pit2 abolished receptor function for both viruses (22).
FeLV-B and GALV do, however, not entirely share the same receptor
requirements, in that they exhibit different requirements for region A
sequences (27) and for sequences outside region A, e.g.,
Pit1 region A conferred GALV but not FeLV-B receptor function upon Pit2
(22). Interestingly, the presence of the N-terminal
two-thirds (encompassing loops 1, 2, and 3 and the large intracellular
domain) of Pit1 in addition to Pit1 region A resulted in a functional
FeLV-B receptor, and a chimera in which the entire C-terminal third
(encompassing loops 4 and 5) was Pit1 derived also supported FeLV-B
entry (22). Similar results were obtained by Tailor and
Kabat upon testing comparable Pit1-RatPit2 chimeras for FeLV-B receptor
function (26). Thus, N- or C-terminal Pit1 sequences, in
addition to region A, are necessary to confer FeLV-B receptor
function upon Pit2 (22); however, the exact nature of
these sequences is not known.
For A-MuLV, an N-terminal determinant was recently mapped to loop 2 (11, 12); moreover, loop 4 has also been shown to play a
role in A-MuLV entry (12, 13, 17, 22). Indeed, results
obtained with chimeras between Pit1 and a related protein from
Neurospora crassa, Pho-4, suggest that loop 2 and loop 4 sequences interdependently specify A-MuLV entry (12). For
10A1, a role of both loop 2 and region A sequences in entry has also been indicated (13). Recently, using Pit1-RatPit2 chimeras, Tailor and Kabat investigated the receptor function of vector pseudotypes carrying FeLV-B-A-MuLV SU chimeric proteins
(26). Based on the results obtained, the authors suggested
that wild-type FeLV-B SU proteins interact specifically with Pit1 loop
2 and Pit1 loops 4 and 5 (26).
By making chimeras between Pit1 and Pit2 and testing these for FeLV-B
receptor function, we investigated whether the C-terminal FeLV-B
receptor determinant(s) in loops 4 and 5 could be narrowed down and
whether sequences in Pit1 loop 2 constitute the N-terminal FeLV-B
receptor determinant.
Receptor function of the chimeras shown in Tables
1 and
2 was tested by a
transient-transfection-infection assay essentially as described
elsewhere (22). The constructs pOJ74, pOJ75, pOJ80, and pOJ102 have been described previously (22); the
remaining constructs were made by site-directed mutagenesis using the
Altered Sites II kit (Promega) (10); all constructs are
based on the expression vector pcDNA1ArtkpA
(30). To verify the presence of chimeric protein on the cell
surface, receptor functions for GALV and
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A 13-Amino-Acid Pit1-Specific Loop 4 Sequence
Confers Feline Leukemia Virus Subgroup B Receptor Function upon
Pit2
ABSTRACT
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for selected constructsfor 10A1 were tested. GALV (SEATO) pseudotypes (5 × 105
CFU/ml) were produced by PG13GBN cells (14, 15). 10A1
pseudotypes (106 CFU/ml) were obtained by infection of an
NIH 3T3 cell clone harboring the G1BgSvN vector (reference
14 and unpublished data) with viruses derived from
the plasmid pRR151, which encodes Moloney Gag-Pol and 10A1 Env
(21). All pseudotypes were titrated on D17 cells
(22). FeLV-B receptor function was determined on Mus dunni tail fibroblasts (MDTF) (ATCC CRL-2017) using two rounds of
exposure to FeLV-B pseudotypes with a titer of 6 × 102 CFU/ml. The FeLV-B pseudotypes used carried the
LacZ-encoding vector LNPOZ (1) and were obtained by
infecting a D17 cell clone harboring LNPOZ (22) with viruses
derived from feline 355-511 cells transfected with a pBR322
clone of the 
HF60 insert of FeLV-B (Gardner-Arnstein [G/A])
(4, 6, 18); FeLV-B(G/A) was a kind gift from Joyce Dunn.
TABLE 1.
Permissivity for infection in cells transiently
expressing Pit1, Pit2, or chimerasa
TABLE 2.
Permissivity for infection in MDTF transiently expressing
Pit1, Pit2, or chimerasa
A stretch of 13 Pit1-specific amino acids in loop 4 confers FeLV-B
receptor function upon Pit2.
Previous results have shown that the
presence of Pit1 region A in Pit2 was not sufficient to confer FeLV-B
receptor function upon Pit2, while the presence of the entire
C-terminal third of Pit1 comprising loops 4 and 5 resulted in FeLV-B
receptor function (22, 26). Region A (Pit1 residues 550 through 558) contains seven of the nine amino acid differences between
Pit1 and Pit2 in loop 4, with the remaining two residues positioned
immediately upstream of region A (Pit1 positions 546 and 548 [Fig.
1]). In accordance with previous
results, we found that a Pit2-derived chimera harboring Pit1 region A
did not support FeLV-B entry (pOJ80 [Tables 1 and 2]). However, the
chimera A7, which differs from pOJ80 by the presence of the
Pit1-specific residues, Y and V, in positions W518 and I520 (Pit2
numbering), respectively, supported FeLV-B entry at a level of
approximately 60% of that obtained on cells expressing Pit1 (Table 2;
Fig. 1). In line with this observation, all constructs harboring an
entire Pit1 loop 4 sequence (A6, A7, A8, and A9 [Table 2]) supported
FeLV-B entry, while no chimera harboring only a Pit1-specific region A
sequence did so (A3, A10, A14, and pOJ80 [Tables 1 and 2]). Thus, we
have mapped a C-terminal determinant of FeLV-B receptor function to a
13-amino-acid stretch in loop 4 comprising Pit1 residues 546 through
558. This observation is in agreement with Pit1 region A being
sufficient to confer FeLV-B receptor function upon MusPit1 and the
C-terminal third of Pit1 being sufficient to confer FeLV-B receptor
function upon Pit2 and RatPit2, in that these three chimeras all harbor
the Pit1-specific 13-amino-acid stretch identified here (Fig. 1)
(22, 26, 27).
|
and possibly
10A1receptor function (5, 8, 12, 13, 17, 22, 23, 25-27),
the Pit1-specific residues in positions 546 and 548 have not previously
been shown to be involved in receptor function for viruses utilizing
Pit proteins as receptors. Whether all of the 13 amino acids in loop 4 here identified are necessary for conferring FeLV-B receptor function
upon Pit2 is not known. Also, infection studies as employed here do not
allow for determining whether this sequence provides a binding site for
FeLV-B SU or, e.g., allows the viral SU to interact with other regions
of the receptor.
Pit1-specific loop 2 and region A sequences do not confer FeLV-B receptor function upon Pit2. Although Pit1 region A is not sufficient to confer FeLV-B receptor function upon Pit2, a chimera harboring the N-terminal two-thirds of Pit1 comprising loops 1, 2, and 3 and the large intracellular domain together with Pit1 region A was an efficient FeLV-B receptor (chimera pOJ103 in reference 22). For A-MuLV and 10A1, results have indicated that an N-terminal determinant of receptor function can be mapped to loop 2 (11-13); moreover, a model has been proposed in which FeLV-B variable region B (VRB) interacts with Pit1 loop 2 (26). In an attempt to identify the N-terminal determinant of FeLV-B receptor function in the chimera encoded by pOJ103, we have investigated the ability of Pit1 loop 2 to confer receptor function upon pOJ80 (Tables 1 and 2). The presence of the most divergent C-terminal part of Pit1 loop 2 (pA3 [Table 1]) or of the entire Pit1 loop 2 sequence (pA14 [Table 2]) did not confer receptor function upon pOJ80. Thus, since pOJ103 is an efficient FeLV-B receptor (22), we conclude that Pit1 loop 2 does not constitute the N-terminal determinant of FeLV-B receptor function; however, whether it is part of a larger N-terminal determinant cannot be determined.
A two-step model for the interaction between FeLV-B SU and Pit1 was recently put forward by Tailor and Kabat, the authors suggesting a specific and discrete interaction of FeLV-B VRA with Pit1 loops 4 and 5 and of FeLV-B VRB with Pit1 loop 2 (26). The proposed two-step model cannot explain the observation that FeLV-B can utilize chimeras harboring Pit1 loops 4 and 5 together with Pit2 or RatPit2 loops 1, 2, and 3 and the large intracellular domain (22, 26) or, as shown here, a Pit2-derived chimera harboring only Pit1 loop 4 residues in positions 518 through 530 (pA7 [Table 2]). All these chimeras are efficient FeLV-B receptors; thus, a Pit1-specific loop 2 sequence is not essential for FeLV-B receptor function in chimeras harboring the 13 amino acids in loop 4 identified here. Moreover, in line with this, a Pit1-specific loop 2 sequence was not sufficient to confer FeLV-B receptor function upon a Pit2-derived chimera already harboring Pit1 region A (chimera A14 [Table 2]). The presence of Pit1 region A was sufficient to confer receptor function for GALV upon Pit2 (pOJ80 in Tables 1 and 2) (22), while FeLV-B receptor function required the presence of other N- or C-terminal Pit1-specific sequences (22). These observations led to the impression that FeLV-B might have somewhat different receptor requirements than those of GALV (22). However, we here found that for both FeLV-B and GALV the same limited protein region confined within a stretch of 13 amino acids in loop 4 is sufficient to confer receptor function upon Pit2. From previous results of our work and others' work and from the results presented here, it is clear that FeLV-B receptor function is only dependent on Pit1-specific N-terminal sequences when the four amino acids upstream of region A are Pit2 specific (22, 26). Interestingly, the ability of loop 4 sequences to confer GALV receptor function was also shown to be context dependent: e.g., while certain region A mutants were compatible with receptor function in a Pit1 background, they only very inefficiently supported GALV entry when placed in a Pit2 background, although the presence of a wild-type Pit1 region A sequence in Pit2 results in an efficient GALV receptor (2). Thus, FeLV-B and GALV have the common trait that sequences in loop 4 are critical for their receptor functions and that certain loop 4 sequences have specific backbone requirements in order to allow receptor function.| |
ACKNOWLEDGMENTS |
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We thank Maribeth V. Eiden for the PG13GBN cell line and Alan Rein for the pRR151 plasmid. We furthermore thank Bente Andersen for excellent technical assistance.
This work was supported by the Karen Elise Jensen Foundation, the Danish Medical Research Council (9802349), NASTRA (540011047), the Novo Nordisk Foundation, and the Danish Biotechnology Programme.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Molecular and Structural Biology, University of Aarhus, C. F. Møllers Allé, Bldg. 130, DK-8000 Aarhus C, Denmark. Phone: 45 8942 2633. Fax: 45 8619 6500. E-mail: LP{at}mbio.aau.dk.
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Journal of Virology, March 2000, p. 2926-2929, Vol. 74, No. 6
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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