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Research Interests
Our laboratory studies the structure and function of the Smith proteins
and their pathology as major autoantigens in Systemic Lupus Erythematosus
(SLE). Our approaches to this problem are to investigate 1) The cytoplasmic
assembly of the Sm proteins with snRNA to form the Sm core particle
2) The disassembly and degradation of the Sm proteins and 3) The genomics
of the larger family of LSm proteins. Throughout this work we ask:
What is unusual about the Sm proteins that makes them vulnerable to
becoming autoantigens in SLE?
1. SnRNP proteins are the Systemic Lupus Erythematosus Sm autoantigens
Autoimmune antibodies in 30% of the patients with systemic lupus erythematosus
(SLE) recognize the Sm proteins, subunits of the heteroheptameric
ring shared by small nuclear ribonucleoprotein (snRNP) particles in
the cell nucleus, as autoantigens. SLE is now recognized as a syndrome,
a set of malfunctions of the immune system resulting in the antigen
driven production of autoantibodies. Susceptibility to SLE maps to
multiple genes in the immune system and includes defects in antigen
clearance. Environmental or stochastic events may also contribute
to the onset. The major autoantigens recognized in SLE are single
and double stranded DNA and the Sm proteins. We hypothesize that the
Sm proteins are prone to becoming autoantigens because they are extremely
abundant and exist in a single complex that has three properties:
powerful B cell epitopes, T cell epitopes and immunostimulatory RNA
species. The major B cell epitopes are the
C-termini of the SmB, SmD1 and SmD3 proteins that have the rare post-translational
modification of symmetrical dimethylarginine (sDMA). The T cell epitopes
stimulating the anti-Sm antibody response reside in the highly conserved
beta strands of the Sm fold. And the snRNA component of the particles
has extensive double stranded regions that can stimulate antigen-presenting
cells through interaction with Toll-like receptor 3. One characteristic
of SLE is lymphopenia. We hypothesize that the over stimulated immune
system suppresses the antibody response, leading to the apoptotic
death of lymphocytes. Then, as a result of defects in normal clearance,
snRNP particles released from the dying lymphocytes stimulate the
anti-Sm response, maintaining the diseased state through positive
feed back mechanism. The initial antibody response against the snRNP
particle may begin thru different paths, but in vulnerable individuals
it spreads to the abundant Sm proteins which in turn perpetuate the
response. Our goal is to identify the specific features of the snRNP
particle that stimulate the immune response. Blocking the immune recognition
of these determinants could inhibit autoantibody production and the
pathology of SLE.
2. Sm proteins assemble with snRNA in the cytoplasm to form the
snRNP core particle
The Sm rings assemble with snRNA in the cytoplasm from three RNA-free
precursors. The three non-Sm proteins 1) pICln, 2) PRMT5 and 3) SMN
assist with the assembly. The Sm ring precursors in the cytoplasm
are 1) a 6S complex of SmD1D2FEG and pICln 2) a 20S complex which
includes SmB, SmD3, SmD1, PRMT5 and pICln, and 3) a 2S-7S complex
that includes the SmB and SmD3 proteins with pICln. PRMT5 methylates
arginines found in RG repeats in the C-termini of SmB, SmD1 and SmD3.
Immunofluorescent studies indicate the RNA-free complexes are distributed
uniformly throughout the cytoplasm and can rearrange in response to
metabolic inhibitors. During core particle assembly, snRNA binds SmBD3
followed by the SmD1D2FEG pentamer to form the ring. Final assembly
occurs in a 60S complex which includes the SMN complex and PRMT5 and
then snRNP particle moves into the nucleus. pICln functions as a chaperone
that interacts with RNA-free Sm protein complexes but not the assembled
Sm rings. The pICln protein has limited homology to the Sm fold and
we hypothesize it interacts with the exposed interfaces of the Sm
proteins. The SmB, SmD1 and SmD3 RNA-free Sm proteins are in equilibrium
between the different complexes in the cytoplasm, however once bound
to the snRNA, they are permanently assembled.
3. Sm proteins are part of the larger family of LSm proteins
that have the Sm fold
The heteroheptameric ring of Sm proteins that binds the snRNAs is
highly conserved throughout eukaryotes. Ten different Sm proteins,
transcribed from nine genes, build four different Sm rings. The Sm
ring is also homologous to the LSm rings that degrade mRNA from the
5’ end in the cytoplasm and which bind a diverse set of RNAs,
including the U6 snRNA, in the nucleus. The Sm and LSm proteins are
characterized by the Sm fold, built from five anti-parallel beta strands
with a helix stacked on top, which forms a subunit of the rings. The
interaction between the subunits is centered on an anti-parallel interaction
of the fourth and fifth beta strands in neighboring subunits and includes
many other contacts between the subunits. The snRNAs wrap around the
inner lumen of the ring, with one nucleotide bound to each of the
Sm subunits. The nucleotide base is inserted into a pocket created
by the loops between _-strands 3 and 4 and _-strands 4 and 5. |
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