Biomedical Engineering Reference
In-Depth Information
in crucial cellular processes such as control of the cell cycle, transcriptional
activation and signaling.
2-5
The occurrence of intrinsically disordered proteins
(IDPs) in disease states is becoming increasingly evident.
6
The occurrence, function and propensity of IDPs to form structured states in
the presence of different partners have been the subject of a lot of recent
research, as well as a large number of review articles on various aspects of the
problem. It is beyond the scope of this brief survey to include references toall
of the work in this field. Review articles
7-10
give overviews of various aspects
of this increasingly active field.
5.2 NMR Methods Used for Disordered Proteins
Solution NMR methods used for folded proteins and complexes are readily
adapted for use with IDPs.
11
When the IDP is completely unfolded, the
resonance lines can be quite narrow, an advantage for NMR experiments that
use long pulse sequences. However, IDPs may contain partly folded, molten
globular or other problematic regions, which can cause loss of NMR signals
(see later section). A number of new NMR techniques have recently been
developed specifically for the use with IDPs, primarily to overcome the
problems of resonance overlap
12
and resonance broadening due to inter-
mediate timescale exchange processes.
13,14
NMR-detected paramagnetic relaxation enhancement (PRE) is frequently
used to obtain structural and dynamic information on disordered proteins. The
magnetic dipolar interaction between an unpaired electron and the nucleusof
interest has a r
26
distance dependence, but the range of the effect is much
greater than, for example, the NOE, because the magnetic moment of the
unpaired electron is much larger than that of the nuclear spin. Since PRE can
estimate distance ranges in conformational ensembles, it is an ideal method for
the characterisation of lowly populated states, for example, in an IDP witha
local conformational preference.
15,16
Residual dipolar couplings (RDCs) have
also been employed for the NMR characterisation of IDPs,
17-20
sometimes in
combination with PRE experiments.
21-23
IDPs by definition are highly dynamic, and the use of NMR dynamics for
IDPs has recently been reviewed.
24,25
The model-free approach
26
is frequently
applied to globular proteins, but several of the assumptions made in this
approach, such as the assumption of a uniform overall correlation time for all
molecules, are invalid for IDPs. Even for proteins that are only partially
disordered, with a structured domain and an unstructured tail, anomalous
dynamics are observed for the structured domain
27,28
apparently due to the
'tail wagging the dog'.
29
5.3 Coupled Folding and Binding
The NMR experiments that originally prompted the idea of functional
intrinsically disordered proteins involved the observation of the formation of
