Untitled - D Ank Unlimited

Disulfide
bonds β pleated sheets
Cysteine
Oligosaccharide
Structure of J chain that occurs in secretory IgA and IgM molecules and
facilitates polymerization.
J (joining) chain
A 17.6-kDa polypeptide chain present in polymeric immunoglobulins,
including IgM and IgA. A diminutive acidic
polypeptide that can bind to tail pieces of α and μ Ig heavy
chains, stabilizing polymeric IgA or IgM molecules. It
links four-chain immunoglobulin monomers to produce the
polymeric immunoglobulin structure. J chains are produced
in plasma cells and incorporated into IgM or IgA molecules
prior to their secretion. Incorporation of the J chain appears
essential for transcytosis of these immunoglobulin molecules
to external secretions. The J chain comprises 2 to 4%
of an IgM pentamer or a secretory IgA dimer. Tryptophan
is absent from both mouse and human J chains. J chains
are composed of 137-amino-acid residues and a single,
complex, N-linked oligosaccharide on asparagine. Human
J chain contains three forms of the oligosaccharide that
differ in sialic acid content. The J chain is fastened through
disulfide bonds to penultimate cysteine residues of μ or α
heavy chains. The human J chain gene is located on chromosome
4q21, whereas the mouse J chain gene is located on
chromosome 5.
J exon
A DNA sequence that encodes part of the third hypervariable
region of a light or heavy chain located near the 5′ ends
of the κ, λ, and γ constant region genes. An intron separates
the J exon from them. The J exon should not be confused
with the J chain. The H constant region gene is associated
with several J exons. The V region gene is translocated to a
site just 5′ to one of the J exons during stem cell differentiation
to a lymphocyte.
J
J gene segment
DNA sequence that codes for the carboxyl terminal 12 to
21 residues of T lymphocyte receptor or immunoglobulin
polypeptide chain variable regions. Through gene rearrangement,
a J gene segment unites either a V or D gene
segment to intron 5′ of the C gene segment.
J region
The variable part of a polypeptide chain consisting of a
T lymphocyte receptor or immunoglobulin that a J gene
segment encodes. The J region of an immunoglobulin light
chain is composed of the third hypervariable region carboxyl
terminal (1 or 2 residues) and the fourth framework region
(12 or 13 residues). The J region of an immunoglobulin heavy
chain is composed of the third hypervariable region carboxyl
terminal portion and the fourth framework region (15 to 20
residues). The J region of the heavy chain is slightly longer
than that of the light chain. The variable region carboxyl
terminal portion represents the J region of the T cell receptor.
JAK-3 SCID
Severe combined immune deficiency (SCID) attributable
to JAK-3 deficiency. Immunophenotypic and functional
analysis of circulating lymphoid cells permits classification
of SCID into distinct subgroups. Neither immunological nor
clinical features can distinguish between X-linked and (AR)
T–B + SCID. Cytokine receptors that utilize the common γ
chain always associate with the intracellular tyrosine kinase
designated JAK-3. The major cytokine receptor transducing
subunit binds to another JAK kinase, JAK-1. Markedly
reduced expression of JAK-3 protein due to mutations in the
JAK-3 gene in unrelated infants with T–B + SCID reveals the
critical role of JAK for lymphoid development and function.
Fewer than 10 SCID patients in the United States are characterized
by defective expression of JAK-3; the incidence
has been greater in Europeans.
JAK–STAT signaling pathway
A signaling mechanism induced by the binding of cytokine
to type I and type II cytokine receptors. Sequential
activation of receptor-associated Janus kinase (JAK)
tyrosine kinases, JAK-mediated tyrosine phosphorylation
of cytokine receptor cytoplasmic tails, docking of signal
transducers and activators of transcription (STATs) to
phosphorylated receptor chains, JAK-mediated tyrosine
phosphorylation of the associated STATs, dimerization and
nuclear translocation of the STATs, and binding STAT to
regulatory regions of target genes leading to transcriptional
activation of those genes all occur.
Janus kinases (JAKs)
Nonreceptor tyrosine kinases that transduce activating
signals from cytokine receptors with which they
associate and phorphorylate STAT transcription factors.
Tyrosine kinases that associate with cytoplasmic tails of
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J