PROCAARE: BASIC SCIENCE--Pathogenesis--Cofactor

[Albert Shaw <ashaw@usa.healthnet.org>]

KEYWORDS: PATHOGENESIS/COFACTOR/G PROTEIN/FUSIN/CHEMOKINE
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Reference: Feng, Y., Broder, C.C., Kennedy, P.E., Berger, E.A. (1996).  
HIV-1 entry cofactor: functional cDNA cloning of a seven-transmembrane, G 
protein-coupled receptor.  Science 272: 872-7.

This report describes the expression cloning of a putative cofactor for 
HIV-1 entry into target cells.  Feng et al. employed a vaccinia 
virus-based system to select for candidate proteins which, together with 
CD4, facilitate viral internalization.  To mimic infecting virus, NIH3T3 
fibroblasts expressed a vaccinia virus-encoded HIV-1 surface env protein; 
in addition, these cells contained a reporter gene (the E. coli lacZ 
gene) linked to a T7 phage-derived promoter.  These NIH3T3 fibroblasts 
were mixed with additional NIH3T3 cells expressing vaccinia virus-encoded 
CD4 and T7 RNA polymerase; this second group of fibroblasts was 
transfected with a cDNA library derived from human HeLa cells, known to 
be permissive for HIV-1 infection.  Thus, only those NIH3T3 cells 
expressing a cDNA coding for a putative cofactor will collaborate with 
CD4 to allow fusion with env protein and activation of the reporter lacZ 
gene by T7 polymerase.

Through successive rounds of cDNA transfection and selection of positive 
cDNA pools, a single clone of 1659 base pairs was identified which 
encoded a 352 amino acid protein; Feng et al. named this protein 
"fusin".  They subsequently demonstrated that transfection and expression 
of fusin in human, monkey and mink cell lines expressing CD4 conferred 
the ability to fuse with cells expressing HIV-1 env protein.  In 
addition, a mink cell line (Mv 1 Lu) expressing CD4 and fusin was able to 
be infected by HIV-1.  Finally, antibodies to fusin were able to block 
HIV-1 infection of human peripheral blood mononuclear cells.

Analysis of the protein sequence of fusin reveals it to contain seven 
putative transmembrane segments; fusin appears to be a member of the 
superfamily of G protein-coupled receptors, and has 37% amino acid 
identity with the interleukin-8 (IL-8) receptor.

This paper provides compelling evidence for fusin as a long-sought 
cofactor for HIV-1 infection.  New lines of research into treatment are 
opened by these results, and in addition, the potential to generate 
improved animal models for HIV-1 infection (e.g. in mice) may enhance 
subsequent studies.  Fusin's intriguing homology to the IL-8 receptor, 
together with the recent implication of the chemokines RANTES, MIP-1 
alpha, and MIP-1 beta in the suppression of HIV-1 infection (Science 270: 
1811) raise important questions on pathogenesis as well as therapy.  
Because fusin was tested against an HIV-1 strain maintained in vitro, the 
authors suggest that other related cofactors may be found for other cell types
and for other HIV-1 strains found in patients.