Biomedical Engineering Reference
In-Depth Information
Figure 5.9
Backbone
1
H-
15
N NOE and effective t
m
values for free (orange and light
blue) and DNA-bound (red and dark blue) +KTS (squares) and -KTS
(circles) ZF1-4 of the Wilms tumor protein, as a function of residue
number. For each zinc-finger domain, the b-hairpin (open arrows) and a-
helix (open rectangles) is indicated, together with linker regions (zigzag
lines) and the alternative KTS splice site (vertical blue bar). Open symbols
indicate resonances that are overlapped and which therefore have large
uncertainties in the relaxation parameters. Adapted from ref. 83 with
permission. # National Academy Sciences, 2000.
bound state they are rigid.
83
The molecular basis for this loss of flexibility in
the linker sequences is the formation of a DNA-induced a-helix cap when the
protein is bound to the correct DNA sequence.
84
Thus it could be envisaged
that the zinc-finger protein could be associated with a long stretch of DNA,
perhaps even bound at low affinity, but readily dissociated from non-cognate
sites to continue searching for the cognate site. Once at the cognate site, the
affinity of the complex is greatly increased by the protein-protein interactions
that result in the structuring of the linkers. This process has been likenedtoa
'snap-lock',
84
and provides a satisfying explanation for the sequence homology
of zinc-finger linkers at the start of fingers 2 and 3 but not of finger 1.
5.5.2 Effects of Alternative Splicing
Alternative splicing of mRNA provides an avenue for diversity of function in a
particular protein, and contributes to conformational diversity that can lead to
evolutionary change. Addition or excision of a peptide segment as a result of
alternative splicing is expected to increase the likelihood of disorder in one or
more splice variants.
85
The variation in the degree of disorder between two
well-known splice variants of the Wilms' tumor zinc-finger proteins was
demonstrated by an NMR study
83,84,86
. The Wilms' tumor zinc-finger protein
