H. sapiens SCP1
Protein
Grace Potter '24 and Nafeez Ishmam Ahmed '23
Contents:
I. Introduction
The H. sapiens SCP1 is a phosphatase enzyme that
mediates the expression of RE1-silencing transcription protein (REST).
REST is a protein involved in silencing neuronal gene expression by
recruitment of transcriptional coregulators, including SCP1. It is
estimated that up to 10% of neuronal genes are regulated by REST.
SCP1 functions by removing a phosphate group from the degron
region of the C terminal domain of REST, which increases stability
and decreases the rate of degradation of REST. There are two
phosphorylated Serines in REST that can be the target of this
dephosphorylation, pSer-861 and pSer-864. SCP1 preferentially
dephosphorylates Ser-861 due to surrounding amino acids. SCP1s
function as a phosphatase is interesting in itself, but its ability
to moderate REST is important. REST has been found to play many
roles in tumor suppression, Alzheimers, and other neurological
diseases. SCP1s ability to knock down expression of REST provides an
important path to learning more about RESTs involvement in these
different diseases, and provides a potential
medication/treatment.
II. General Structure
SCP1 is a heterodimer containing 2 chains, A
and B, that interact with the REST
polypeptide chain.
Each chain is 180 amino acids, and the total length is 360 amino
acids. The total structural weight is 44.4 kDa. Also featured in
this are two-12 amino acid chains that represent small pieces of the
REST polypeptide, chains C and D.
In one of these chains, both Ser861
and Ser864 are phosphorylated. In the
other, only Ser861 is phosphorylated. In
order to capture the image of this protein and the polypeptide
without them actually interacting, they replaced Asp96 with Asn96
.
III. Molecular Interactions
There is a hydrophobic pocket in
SCP1 which is responsible for recognition
of the REST polypeptide sequence.
Important residues in this interaction are Leu860
of the REST protein, and Phe106, Val127
and Tyr158 of SCP1
. The tight hairpin structure of the REST polypeptide allows
the chain to conform to the binding site. Hydrogen bonds between the
carbonyl of Pro862 and the amide group
of pSer864, the carbonyl of Pro863 and
a side chain of Asn187, and the
phosphate group of pSer864 and the carbonyl of Gly186
are specifically responsible for this hairpin
. An interaction between magnesium ions,
Asn96, Asn207 and two neighboring water molecules keep the REST
polypeptide connected to the rest of SCP1.
SCP1 favors the dephosphorylation of Ser861 over Ser864 due to the
limiting nature of the prolines on either side of Ser864. This
series of amino acids limits the flexibility of the chain around
Ser864, and makes it an unfavorable dephosphorylation site.
IV. Function
Because SCP1 is important in regulating the expression of
REST, it is thought that SCP1 regulation could lead to an uptick
in expression of genes restricted by REST. SCP1 restriction using
sRNAs is an easier way to change expression levels of REST
controlled genes than REST restriction because REST is such a
large protein, especially in comparison to SCP1. This graph
represents the relative expression of genes restricted by REST
under SCP1 restriction and under REST restriction. Although the
expression of these genes is slightly lower when controlled by
SCP1 restriction vs REST restriction, the expression is still
relatively high and shows that SCP1 can be used to regulate REST
expressed gene expression.
Testing on the genes USP37 and BEX1 (tumor suppressors),
BDNF and UCHLF (both critical in neurodevelopment), DYRK1A
(associated with tumors), and TUBB3 (a control) showed that SCP1
knockdown is effective in the regulation of REST expressed genes.
Fig. 1. The histograms show relative REST regulated gene
expression with A) shREST knockdown vectors, and B) shSCP1 knockdown
vectors (Burkholder et al. 2018).
V. Discussion
Controlling SCP1's dephosphorylation of Ser861 in REST,
offers a possible preventative for certain cancers and
neurological diseases. Because REST is such a large complex, it is
unlikely that a single siRNA could prevent it from functioning.
SCP1 regulates REST expression through dephosphorylation, as shown
in Fig 2. SCP1 is also smaller and can be targeted by siRNA's, and
as Fig 1 suggests, SCP1 is effective in inhibiting REST
expression. This offers an alternative method of regulating REST.
Fig. 2. Model of SCP1 phosphoregulation of REST function. A,
when REST is phosphorylated at Ser-861 and/or Ser-864, the SCF B-TRCP
complex targets the Ser-1024/27/30 degron and ubiquitinates REST for
degradation. Dephosphorylation at the Ser-861 and Ser-864 sites of
REST by SCP1 prevents the SCF B-TRCP complex from stably associating
with the Ser-1024/27/30 degron. Unphosphorylated REST recruits various
regulatory factors to RE1 elements and represses neuronal gene
expression (Burkholder et al. 2018).
VI. References
Ballas, Nurit, et al. 2005. REST and its corepressors mediate
plasticity of neuronal gene chromatin throughout neurogenesis. Cell.
121: 645-657.
Burkholder, Nathaniel Tate, et al. 2018. Phosphatase activity
of small C-terminal domain phosphatase 1 (SCP1) controls the
stability of the key neuronal regulator RE1-silencing
transcription factor (REST). Journal of Biological
Chemistry. 293(43): 16851-16861.
Otto, Stephanie J., et al. 2007. A new binding motif for the
transcriptional repressor REST uncovers large gene networks
devoted to neuronal functions. The Journal of Neuroscience.
27(25): 6729-6739.
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