PUTATIVE ALPHA-HELICAL STRUCTURES IN THE HUMAN-IMMUNODEFICIENCY-VIRUSTYPE-1 VPU PROTEIN AND CD4 ARE INVOLVED IN BINDING AND DEGRADATION OFTHE CD4 MOLECULE
E. Tiganos et al., PUTATIVE ALPHA-HELICAL STRUCTURES IN THE HUMAN-IMMUNODEFICIENCY-VIRUSTYPE-1 VPU PROTEIN AND CD4 ARE INVOLVED IN BINDING AND DEGRADATION OFTHE CD4 MOLECULE, Journal of virology, 71(6), 1997, pp. 4452-4460
The human immunodeficiency virus type 1 (HIV-1) vpu gene encodes a 16-
kDa class I integral membrane phosphoprotein with an N-terminal membra
ne-spanning region and a C-terminal cytoplasmic domain, In the cytopla
smic domain, two amphipathic alpha-helices joined by a flexible turn c
ontaining two phosphoacceptor sites have been predicted, Previous stud
ies have shown that Vpu downregulates CD? molecules by inducing their
specific degradation in the endoplasmic reticulum, Phosphorylation of
serine residues 52 and 56, present within the cytoplasmic domain of th
e Vpu protein, has been shown to be essential to this Vpu function, Ho
wever, the contribution of these two phosphoacceptor sites in the mech
anism of CD4 degradation remains undefined, Interestingly, a specific
interaction between Vpu and CD4 was recently demonstrated in coimmunop
recipitation experiments, Binding of Vpu was shown to be necessary but
not sufficient to mediate CD4 degradation, indicating that interactio
n between Vpu and CD4 represents an early step critical in triggering
a process leading to CD4 degradation, To delineate the sequence(s) and
/or structural determinant(s) involved in this Vpu-CD4 interaction and
in the Vpu-mediated CD4 degradation, we performed a mutational analys
is of the cytoplasmic domain of CD4 and Vpu, Coimmunoprecipitation exp
eriments reveal that disruption of the putative alpha-helical structur
e in the membrane-proximal cytoplasmic domain of CD4 affects the bindi
ng to Vpu, suggesting that this structure may act as an interface for
the CD4-Vpu interaction that mediates CD4 degradation, Vpu proteins co
ntaining mutations in either or both of the phosphoacceptor sites (Ser
52 or/and Ser56) were inactive in regard to CD4 degradation yet retain
ed the capacity to interact with the cytoplasmic domain of CD4, In an
attempt to define the minimal region responsible for this interaction,
we tested a panel of mutations which were designed to affect the inte
grity of the putative alpha-helices present in the cytoplasmic domain
of Vpu, Our results indicate that although both C-terminal alpha-helic
es are required for degradation of CD4, only alpha-helix I, located in
the membrane-proximal cytoplasmic region of VPu, is involved in the i
nteraction between Vpu and CD4, Taken together, these results demonstr
ate that alpha-helical structures in the HIV-1 Vpu and CD4 proteins ar
e involved in binding and degradation of CD4 molecules.