Biomedical Engineering Reference
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exist which contain a number or a number and letter, e.g. IFN-
α
7, I F N-
α
8, IFN-
α
2B. Just to ensure
total confusion, several are known by two different names, e.g. IFN-
α
7 is also known as IFN-
α
J1.
8.2.2 Interferon-
β
IFN-
, normally produced by fi broblasts, was the fi rst interferon to be purifi ed. Humans synthesize
a single IFN-
β
molecule, containing 166 amino acid residues, that exhibits 30 per cent sequence
homology to IFN-
β
s. The mature molecule exhibits a single disulfi de bond and is a glycoprotein
of molecular mass in excess of 20 kDa. The carbohydrate side chain is attached via an N-linked
glycosidic bond to asparagine residue 80. The carbohydrate moiety facilitates partial purifi cation
by lectin affi nity chromatography. Immunoaffi nity chromatography using monoclonal antibodies
raised against IFN-
α
β
, as well as dye affi nity chromatography, has also been employed in its pu-
rifi cation. IFN-
helical segments, three of which lay
parallel to each other, with the remaining two being antiparallel to these.
β
's tertiary structure is dominated by fi ve
α
8.2.3 Interferon-
γ
IFN-
is usually referred to as 'immune' interferon. It was initially purifi ed from human periph-
eral blood lymphocytes. This interferon is produced predominantly by lymphocytes. Its synthesis
by these cells is reduced when they come in contact with presented antigen. Additional cytokines,
including IL-2 and -12, can also induce IFN-
γ
γ
production under certain circumstances. A sin-
gle IFN-
gene exists, located on human chromosome number 12. It displays little evolutionary
homology to type I interferon genes. The mature polypeptide contains 143 amino acids with a
predicted molecular mass of 17 kDa. SDS-PAGE analysis reveals three bands of molecular mass
16-17, 20 and 25 kDa, arising because of differential glycosylation. The 20 kDa band is glyco-
sylated at asparagine 97, whereas the 25 kDa species is glycosylated at asparagines 25 and 97. In
addition, mature IFN-
γ
exhibits natural heterogeneity at its carboxyl terminus due to proteolytic-
processing (fi ve truncated forms have been identifi ed). The molecule's tertiary structure consists
of six
γ
-helical segments linked by non-helical regions.
Gel-fi ltration analysis reveals bands of molecular mass 40-70 kDa. These represent dimers (and
some multimers) of the IFN-
α
polypeptide. Its biologically active form appears to be a homodimer
in which the two subunits are associated in an antiparallel manner.
γ
8.2.4 Interferon signal transduction
All interferons mediate their biological effect by binding to high-affi nity cell surface receptors. Bind-
ing is followed by initiation of signal transduction, culminating in an altered level of expression of
several interferon-responsive genes. Although both positive regulation and negative regulation exist,
positive regulation (up-regulation) of gene expression has been studied in greatest detail thus far.
All interferon-stimulated genes are characterized by the presence of an associated interferon-
stimulated response element (ISRE). Signal transduction culminates in the binding of specifi c
regulatory factors to the ISRE, which stimulates RNA polymerase II-mediated transcription of the
interferon-sensitive genes. The induced gene products then mediate the antiviral, immunomodula-
tory and other effects characteristically induced by interferons.
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