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G protein
Proteins that bind guanosine triphosphate (GTP) and convert
it to guanosine diphosphate (GDP) during cell signal
transduction. These heterotrimeric proteins are active when
GTP occupies the guanine binding site and inactive when
it anchors GDP. The two types of G proteins include the
trimeric (α, β, γ) receptor-associated G protein and the
small G proteins, including Ras and Raf, which function
downstream of numerous transmembrane signaling events.
Trimeric GTP-binding proteins are associated with parts of
numerous cell surface receptors in the cytoplasm, including
chemokine receptors.
G protein-coupled receptor family
Receptors for lipid inflammatory mediators, hormones, and
chemokines that employ associated trimeric G proteins for
intracellular signaling.
GAD-65
A major autoantigen in insulin-dependent diabetes mellitus.
It is expressed primarily in human β cells.
gag
The retroviral HIV-1 gene that encodes the heterogeneous
p24 protein of the virus core.
GALT
Abbreviation for gut-associated lymphoid tissues. Refer also
to MALT.
globulin
Obsolete designation for immunoglobulin. γ Globulins are
serum proteins that show the lowest mobility toward the
anode during electrophoresis when the pH is neutral. The γ
globulin fraction contains immunoglobulins. It is the most
cationic of the serum globulins.
globulin fraction
The electrophoretic fraction of serum in which most of the
immunoglobulin classes are found.
heavy chain disease
γ Heavy chain disease, also called Franklin’s disease, is a
very rare syndrome in which the myeloma cells synthesize
γ heavy chains only. Clinically, this disease affects mostly
older individuals; onset is gradual or sudden. The patients
may be weak, have fever and malaise, and demonstrate
lymphadenopathy over time. Swelling of the uvula and
edema of the palate may be consequences of lymphoid tissue
involvement of the nasopharynx and Waldeyer’s ring.
Lymph nodes affected may include those of the axilla,
mediastinum, tracheobronchial tree, and abdomen. Fever
and enlargement of the spleen and liver may follow infection.
The disease may last from several months to 5 years
and usually leads to death, although remission has been
described in occasional patients. The blood serum contains
proteins with electrophoretic peaks that correspond
in mobility to the homogeneous (Bence–Jones negative)
proteins present in the urine. An elevated sedimentation
rate, mild anemia, thrombocytopenia, leukopenia, and
G
sometimes eosinophilia occur. Abnormal plasma cells and
lymphocytes may appear in the blood and occasionally be
manifested as plasma cell leukemia. To confirm the diagnosis
of γ heavy chain disease, one must demonstrate a spike
with the electrophoretic mobility of a fast γ or β globulin
reactive with antiserum against γ heavy chains, but not with
κ or λ light chains. Numerous deletions in γ chains vary in
location but often include most of the variable region and
the total C H1 segment, with continuation of the normal
sequence where the hinge region begins. Whereas patients
with Franklin’s disease demonstrate heavy γ chains in
both serum and urine, the other immunoglobulin levels are
diminished. Light chain synthesis totally fails in all individuals.
Thus, γ heavy chain disease may mimic lymphomas
of one type or another (as well as multiple myeloma,
toxoplasmosis, and histoplasmosis) and may be associated
occasionally with tuberculosis, rheumatoid arthritis, and
various other conditions.
interferon
Refer to interferon γ.
macroglobulin
Obsolete term for immunoglobulin M (IgM).
T cell receptor (TCR)
A heterodimer comprised of TCRγ and δ chains together
with the CD3 complex. Antigens include MICA/MICB,
Hsps, T10/22 proteins, altered phospholipids, lipoproteins,
phosphorylated oligonucleotides, pyrophosphatelike
epitopes, alkylamines, peptides, pathogens, or whole
proteins. Selected γδ TCRs can recognize CD1c. Doublenegative
CD4 – CD8 – T cells may express CD3-associated γδ
TCRs. γδ TCRs may be capable of reacting with polymorphic
ligand(s). The initial receptor to be expressed during
thymic ontogeny is the γδ TCR. Cells lacking γδ TCR
expression may subsequently rearrange α and β chains,
resulting in αβ TCR expression. Thus, a single cell never
normally expresses both receptors. Cells expressing the αβ
TCR mediate helper T cell and cytotoxic T cell functions.
Whereas γδ TCRs are expressed on double negative cells
only, αβ TCRs may be present on CD4 + or CD8 + . The γδ
TCR apparently can function in the absence of major histocompatibility
complex (MHC) molecules, in contrast to the
association of CD4 + with MHC class II or CD8 + with MHC
class I recognition by cells expressing αβ TCRs. γδ-TCRexpressing
cells may protect against microorganisms entering
through the epithelium in the skin, lung, intestines, etc.
γδ-TCR-bearing cells constitute a majority of T cells during
thymic ontogeny and in mouse epidermis. Whether the epidermis
and/or epithelium of the skin can function as sites
for T cell education and maturation is unknown. γδ TCR
represents an evolutionary precursor of the αβ TCR, as
reflected by the relatively low percentages of cells expressing
γδ TCRs in adults and the fact that cells expressing the
αβ TCRs carry out the principal immunologic functions.
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