Chemistry Reference
In-Depth Information
Xeroderma pigmentosum
(XP). The minimal factors necessary for removal of
damaged nucleotides also include XPA and RPA which are among the major
damage-recognition proteins involved in the early stages of NER. XPA (32 kDa)
and RPA (70, 34 and 14 kDa subunits) are able to bind damaged DNA independ-
ently, although RPA interaction stimulates XPA binding to damaged DNA.
XPA belongs among proteins that bind preferentially to bent or kinked DNA
duplexes.
50
Interestingly, XPA protein binds with considerable selectivity to the rigid
double-stranded kink induced by the 1,2-GG intrastrand crosslink of cisplatin.
51
In
contrast, XPA fails to recognize the site of strong duplex destabilization generated
by the same crosslink, but formed by the bifunctional dinuclear platinum complex
[{
trans
- PtCl(NH
3
)
2
}
2
H
2
N(CH
2
)
4
NH
2
]Cl
2
51
that fails to impose a rigid kink, but,
instead, increases DNA fl exibility in a nondirectional manner.
52
RPA, on the other
hand, recognizes the helical destabilization caused by the dinuclear complex more
effectively than the cisplatin-induced kink. RPA binds with a relatively low affi nity
to double-stranded DNA containing the 1,2-GG intrastrand crosslink.
53
The prefer-
ence of RPA for the more helix destabilizing crosslink
54
is consistent with previous
studies suggesting that RPA is recruited to damaged double-stranded DNA through
interactions with single-stranded sites.
53,55
It has been suggested
51
that XPA in con-
junction with RPA constitutes a regulatory factor that monitors DNA bending and
unwinding. In addition, it has been demonstrated
56
that the particular manner in
which XPA and RPA proteins align themselves with respect to the 1,3-GNG intras-
trand crosslink involves their binding in close proximity to the adduct, with XPA in
contact with both the platinated and unplatinated strands of DNA, and RPA binding
preferentially to the unplatinated strand. It has also been shown
57
that a direct
interaction between RPA and XPA proteins facilitates the assembly of a preincision
complex during the NER processing of DNA containing a 1,2-GG intrastrand
crosslink. A ternary complex of RPA and XPA both bound to DNA containing the
1,2-GG intrastrand crosslink displays modest specifi city for platinated versus unp-
latinated DNA. In addition, the RPA-XPA complex exhibits a greater affi nity for
binding cisplatin-damaged duplex as compared to the RPA or XPA proteins alone.
It hasalso been demonstrated
57
that the role of XPA is to stabilize the double-
stranded DNA structure by inhibiting the strand separation activity of RPA. XPA
is also believed to participate in DNA damage recognition of cisplatin adducts,
whereas it has no affi nity for DNA adducts of clinically ineffective transplatin.
58
The effects of the lesions induced by single, site-specifi c 1,2 - GG or 1,3 - GTG
intrastrand adducts of cisplatin on the energetics of DNA have been examined as
well.
59
The results have confi rmed that DNA bending is the specifi c determinant
responsible for high-affi nity interactions of XPA with damaged DNA. An additional
important factor is a change of thermodynamic stability of DNA induced by the
damage. These results confi rm that RPA preferentially binds to DNA distorted in
such a way that hydrogen bonds between complementary bases are interrupted.
RPA also binds to nondenatured distortions in double-helical DNA, but the affi nity
of RPA to these distortions is insensitive to alterations of thermodynamic stability
of damaged DNA.
XPC-HR23B displays an enhanced binding affi nity for 1,3-GTG intrastrand
crosslinks compared to unmodifi ed DNA.
60
In addition, XPC-hHR23B associates
