Biomedical Engineering Reference
In-Depth Information
review, or even a collection of reviews, to provide an adequate summary of the
current knowledge about NF-
B. The purpose of this volume is to collect in one
place a number of chapters, written by leading authorities who were involved in
making the seminal discoveries that have shaped this field, in a form that emphasizes
current knowledge, with enough historical information to make the process of dis-
covery apparent. Significant effort has been invested in making the chapters similar
in style, giving this volume the feel of a textbook. It is also anticipated that with
input from the community, this handbook will be updated at regular intervals, to
ensure that the information included remains current and useful.
We begin this handbook on NF-
κ
B by first providing a summary introduction
to the key features of this transcription factor. We believe this information will
provide an uninitiated reader easier access to the complexities in the NF-
κ
B system.
Subsequent chapters will delve in greater detail into different regulatory features of
this transcription factor, culminating with a discussion of current efforts to develop
therapies aimed at inhibiting the aberrant activity of NF-
κ
κ
B in different diseases.
1.2
NF-
κ
B/REL PROTEINS
In the majority of cell types NF-
B exists in the cytoplasm as homo- or heterodimers
of a family of structurally related proteins. Mammalian cells express five members
that contain a conserved N-terminal region called the Rel homology domain (RHD)
within which lies the DNA-binding and dimerization domains and the nuclear
κ
localization signal (NLS) (
Chapter 2
). In unstimulated cells, NF-
κ
B complexes are
sequestered in an inactive form via interaction with a monomer of an inhibitory
protein called I
B, which itself belongs to a structurally and functionally related
family of molecules. Signals that induce NF-
κ
κ
B activity cause degradation of I
κ
B,
allowing NF-
κ
B dimers to translocate to the nucleus and induce gene expression
(
Figure 1.1
).
Although in most cases NF-
κ
B activity must be induced, in certain cell
types, e.g., mature B cells, thymocytes, monocytes, macrophages, neurons, corneal
keratinocytes, vascular smooth muscle cells, and many tumor cells, NF-
κ
B can also
be detected as a constitutively active, nuclear protein.
The first reports of purification of NF-
B DNA-binding subunits to homogeneity
identified two proteins with molecular weights of approximately 50 and 65 kDa
[3,4], respectively. Purification was performed using
κ
B-site-specific DNA affinity
chromatography and the two proteins, referred to as p50 (also called NF-
κ
B1) and
p65 (RelA), were subsequently shown to form heterodimers that bind specifically
to cognate
κ
B/Rel proteins were determined
after molecular cloning of p50, which revealed that the N-terminal 300 amino acids,
subsequently termed the Rel-homology domain, was highly homologous to the
oncogene
v-Rel
, its corresponding cellular homologue c-Rel and the
Drosophila
protein Dorsal [4,5]. Further studies revealed that p65/RelA, p100/p52 (NF-
κ
B-sites. The functional domains of NF-
κ
B2),
and Rel-B were also members of the mammalian Rel-family, thereby bringing the
total to five. Almost every combination of NF-
κ
B/Rel proteins as homo- or het-
erodimers, in either the cytoplasm or nucleus of many different cell types, has been
described. For example, p50/p65 heterodimers are found in the cytoplasm of most
κ
