6S
Complex: Intermediate in snRNP Core Assembly
Ali Fox '21 and Keely Lovato '20
Contents:
I.
Introduction
Splicing, the
process by which introns are removed from pre-mRNA in eukaryotes, is a
fundamental system in molecular biology. Alternative splicing allows a
near limitless number of proteins to be transcribed from a fixed
number of genes. Complex organisms, such as humans, owe their
existence to the proteome diversity afforded by alternative splicing
(Graveley et al. 1996). Many of the necessary spliceosome components
are small nuclear ribonucleoproteins (snRNPs) (Grimm et al. 2013).
Although they
perform different functions, all snRNPs contain a common core
consisting of seven Sm proteins (B, D1, D2, D3, E, F, G) bound to a
distinct snRNA, forming a ring. Formation of the core occurs over
several steps and involves a number of other proteins.
Formation of the core occurs over several steps and involves a number of other proteins. The 6s complex is an early intermediate formed during the stepwise synthesis of the snRNP core that includes SmD1, SmD2, SmE, SmF, SmG, and assembly chaperone pICln. This 6s complex docks onto the Gemin2/SMN complex (forming the 8s complex), which removes pICln, opening the ring of Sm proteins. This allows the complex to be transferred onto snRNA, and for SmD3/B to be added (Grimm et al. 2013).
Simplified
version of core formation:
(1)
Sm [B, D1, D2, D3, E, F, G] = SmD1/D2,
SmE/F/G, SmD3/B *occurs spontaneously (Raker et al.
1996)
(2)
SmD1/D2 + SmE/F/G
+ pICln
= 6s complex
(3) 6s complex
+ Gemin2/SMN
= 8s
(4)
8s = Late Intermediate
+ unbound pICln
(5)
Late Intermediate + snRNA +
SmD3/B
= Core
II.
General Structure
The Sm core
domain is composed of 7 Sm proteins:
SmE-SmG-SmD3-SmB-SmD1-SmD2-SmF. However, in vitro studies have shown
that the proteins first form oligomeric building blocks
and
. Without SnRNA, these oligomers are initially unable to interact with
each other, but assembly chaperone pICln mediates the formation of the
, an intermediate composed of SmE-SmG-pICln-SmD1-SmD2-SmF.
This complex can subsequently dock onto the Gemin2/SMN complex, where pICln is removed, the remaining core is transferred onto SnRNA, and SmD3/B joins the complex to form the full Sm protein core (Grimm et al. 2013).
III.
Sm Structure
have what is called an Sm fold, consisting of a strongly bent five-stranded antiparallel beta sheet and an N-terminal alpha helix.
In both the fully assembled Sm cores and in the 6S Complex, the B4
strand of one Sm protein connects to the B5 strand of the adjacent
Sm protein.
IV.
pICln Structure
adopts
an extended pleckstrin homology (PH) topology. The seven
antiparallel beta strands (B1-B7) and C-terminal alpha helix that
are characteristic of a PH domain have an additional N-terminal Beta
strand (called B0),
which is connected to B1 by an elongated loop and is adjacent to B4
in an antiparallel direction.
PICln is an Sm protein mimic - in the sequence SmE-SmG-SmD3-SmB-SmD1-SmD2-SmF, PICln takes the place of SmD3/B, but the rest of the Sm proteins are in the same position as they are in a fully assembled snRNP, demonstrating that PICln is a topological organizer that sets the sequence of the mature Sm protein complex at an early stage of assembly.
V.
Formation of the 6s Complex
Sm proteins
interact with each other through the formation of hydrogen bonds
between antiparallel beta-sheets. These bonds create a continuous,
circular beta sheet strand that spans the entire 6s complex. pICln
mimics Sm proteins and attatches to SmD1 through
(pICln-B5 and
SmD1-B4 ).
form between residues on pICln-B5
and SmD1-B4.
bond forms between two residues (pICln-Ser66
and SmD1-Gln54)
neighboring the beta-sheets. This interaction is further stablized
by a
Interestingly,
pICln and SmG interact through
, rather than antiparallel. The extended PH fold of pICln allows for
and
to occur between pICln-B0
and SmG-B5. Although
the orientation of this pairing is different, it still aides in the
formation of a continuous beta sheet.
The bonds between the parallel strands of pICln and SmG are thought to be more flexible than between the antiparallel strands that connect the rest of the complex. This makes the pICln-SmG contact a likely point for destabilization when the 6s complex docks onto the SMN/Gemin2 complex and pICln is removed (Grimm et al. 2013)
In addition
to beta-sheet pairing, complementary charges also aids in the
formation of the 6s complex, because Sm proteins have positive
electrostatic potential and pICln has a strong negative potential
(Grimm et al. 2013).
VI.
References
.
Graveley, B. (2001) Alternative splicing: increasing diversity in the proteomic world. Trends in Genetics. 17 (2): 100-107.
Grimm, C., Ashwin, C., Jann-Patrick, P., Kuper, J., Kisker, C., Diederichs, K., Stark, Schindelin, H., and Fischer, U. (2013) Structural Basis of Assembly Chaperone- Mediated snRNP Formation. Molecular Cell. 49 (4): 692-703.
Schultz, Steve C., George C. Shields, and Thomas A. Steitz. (1991) Crystal Structure of a CAP-DNA complex: The DNA Is Bent by 90 degrees. Science. 253: 1001-1007.
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