Biology Reference
In-Depth Information
decade, providing a wealth of detail about the protein side of this cru-
cial interaction. The efforts are largely driven by the global endeavor
to combat acquired immunodeficiency syndrome (AIDS), whose
causative agent is the HIV retrovirus.
37-42
However, related retro-
viruses like the moloney murine leukemia virus (MMLV),
43
mouse
mammary tumor virus (MMTV)
44
and Mason-Pfitzer monkey virus
(MPMV)
45
were also extensively studied. All of these proteins have in
common that they contain one or two copies of a CCHC-type zinc
knuckle motif with the conserved sequence Cys-X
2
-Cys-X
4
-His-X
4
-
Cys, where X stands for a conservatively substituted amino acid. This
sequence/structure motif is directly involved in RNA binding, as
demonstrated by the fact that mutations that interfere with Zn bind-
ing greatly inhibit or abolish genome packaging.
46-50
All of these
zinc knuckles contain a regular pattern of conservatively substituted
hydrophobic and aromatic residues that form a cleft on the surface of
the structure. Substitutions of these amino acids result in altered RNA
packaging specificity, indicating that they are involved in sequence-
specific RNA-protein interactions.
51
The NC protein of human immunodeficiency virus type-1 (HIV-1)
contains two such zinc knuckle domains, which are separated by a short
linker sequence (RAPRKKG). The amino-terminal domain is essential
for genome recognition.
52
The carboxy-terminal domain plays addi-
tional roles in viral assembly and during the early stages of viral infec-
tion.
53,54
The two domains adopt similar three-dimensional structures
with the amino-terminal X-C-X-X-C-X-G-X sequence folding into a
metal-coordinating reverse turn that has been termed “rubredoxin
knuckle” because of its similarity with the structure found in the iron-
coordinating domain of rubredoxin.
42
The subsequent residues form
a loop followed by a 3
10
-helix (Fig. 6).
41,42
The linker sequence shows
little evidence of structure; however, NMR spectroscopic studies
revealed transient interactions between the two zinc-knuckle
domains.
37,38
The overall structure of the NC protein can be best
described by a rapid equilibrium between structures in which the
knuckles interact with each other and where they are independent
from each other. In light of other RNA-protein interactions, it is likely
