Transcript The Structural Basis of Familial Parkinson`s Disease

Kateryna Zorych
The Structural Basis of Familial Parkinson’s Disease
Parkinson’s disease is characterized by tremor, stiffness, and slowing
movement due to the reduced levels of dopamine. Additional to the motor
skills impairment, Parkinson disease is often accompanied by depression,
anxiety, and apathy resulting from the loss of the essential neurotransmitter
dopamine.
Leucine-Rich Repeat Kinase 2 (LRRK2), also known as dardarin, is a protein
linked to Parkinson disease. There are four LRRK2 gene variants commonly
found in patients with Parkinson’s disease (about one-third of the cases) and
is associated with type 8 Parkinson’s disease.
The Structure of Leucine-Rich Repeat Kinase 2
The LRRK2 protein possesses an ankryin repeat region, a leucine-rich
repeat (LRR) domain, a kinase domain, a DFG-like motif, a RAS domain,
a GTPase domain, a MLK-like domain, and a WD40 domain. The protein
is present largely in the cytoplasm but also associates with the
mitochondrial outer membrane.
Here we present the structure of the Ras of complex proteins (ROC)
domain that may act as a GTPase to regulate its protein kinase activity
[1].
ROC Domain of The LRRK2 Forms A Dimeric GTPase
GDP
GDP
Mg2+
Chain B
Mg2+
Chain A
Surface representation highlighting the GDP-Mg2+ binding pocket on the surface
of the dimer that is contributed from both monomers.
Mg2+
Binding
Pocket
GDP
Binding
Pocket
GDP
Mg2+
Structural basis of PD-associated mutations in ROC:
Mutations at the residues important for dimerization or GTPase formation alter
GTPase activity, resulting in pathological consequences. Here, the importance of
the residue Arginine R1441 is explored. The arginine-to-glycine substitution at the
residue is commonly found in Parkinson’s disease patients of Basque ancestry,
[2] while the arginine-to-cysteine mutation is implicated in autosomal dominant
Parkinson’s [3]. Many other Parkinson’s-related LRRK2 mutations have been
identified.
A Drosophila model expressing the human LRRK2 G2019S mutation in neuronal
cells showed adult-onset loss of dopaminergic neurons, locomotor dysfunction,
and early mortality. Treatment of mutant flies with L-dopa improved locomotor
impairment but did not prevent the loss of dopaminergic cells. [4]
The Hydrophobic Zipper at The Dimer Interface is Essential
to Proper LRRK2 Function
Arginine R1441 and tryptophan W1434 from one monomer (chain
A, highlighted in Red) together with phenylalanine F1401 and
proline P1406 from the other monomer (chain B, highlighted in
Cyan) stack on each other alternately, forming a hydrophobic
zipper at the dimer interface [1]. The guanidinium group of R1441
is hydrogen-bonded with the backbone carbonyl oxygen of F1401
and the hydroxyl group of T1404.
Arginine R1441 Interacting with Neighboring Residues
W1434
T1404
F1401
R1441
P1406
H-Bonding
Interactions
A Proposed Mechanism for the ROC Domain of LRRK2 Regulating Its
Kinase Function
LRRK2 is an interesting kinase because its two enzymatic domains within a
single polypeptide communicate to control activity [5, 6], as the kinase
activity of LRRK2 is stimulated upon GTP binding to ROC [7, 8].
Although the mechanism of GTP regulation of kinase activity is not fully
understood, it has been postulated that ROC regulates the kinase activity
by alternating its conformations through a GTP-bound (active) and GDPbound (inactive) cycle, which suggests that loss of binding of GTP or
increasing turnover of GTP to GDP is likely to result in lowered kinase
activity. [1]
Juvenile Parkinsonism: a Devastating Disease
Juvenile parkinsonism is defined by its early onset, appearing in patients younger
than 40.
Familial autosomal recessive Parkinson’s identified in a Japanese family. The
defining features of the disease were unusually early onset, generally at
adolescence, short lifespan, and the absence of Lewy bodies in the brain at
autopsy. [ref 1a]
Another Japanese research group published a study of 12 Japanese families, in
11 of which the individuals affected with Parkinson’s were children from
consanguineous marriages. In these families, the mean age of parkinsonism onset
was 27. Patients responded to levodopa treatment but presented with dopainduced dyskinesias and wearing-off phenomena [ref 2a]
Yet another study identified a possibly new type of early-onset Parkinson’s disease
in which the patients did not respond to levodopa but instead showed
improvement upon treatment with trihexyphenidyl [ref 3a]
Inheritance Of Juvenile Parkinson’s Disease
Juvenile Parkinson’s has been associated with a mutant Park2 gene, found on
chromosome 6.
The mutations are recessive, with homozygous mutations resulting in juvenile
Parkinson’s while the heterozygous mutations are associated with later-onset
Parkinson’s.
In a group of 15 families from 4 distinct ethnic backgrounds, Jones et al. (1998)
[ref 4a] found the locus for autosomal recessive juvenile parkinsonism in all to be
mapped to 6q25.2-q27. Notably, the PARK2 gene maps to 6q25.2-q27, the
region to which autosomal recessive juvenile parkinsonism maps.
Mutations in Parkin and Familial Parkinson’s Disease
The Parkin protein contains an N-terminal ubiquitin-like (UBL) domain and 2
C-terminal RING finger domains that are separated by an in-between ring
(IBR) domain.
The RING-IBR-RING (RBR) structure is highly conserved and can only be
found in eukaryotes. The IBR domain has 2 zinc-binding sites. Zinc binding is
required for the correct folding of the IBR domain, which is necessary for
proper protein interactions and subsequent ubiquitination.
Parkin IBR domain, the 78
Zn2+
Ions
residue stretch (residues
M307–S384) comprising
the IBR domain depicted in
rainbow. The two zinc ions
are indicated in the figure.
The Structure of Parkin In-Between Ring (IBR) Domain:
According to the NMR structure determination [ref5a], folding of the IBR domain
(residues M327–S378) was found to be zinc dependent and the N terminus of
the IBR domain, residues E307–E322, was found to be unstructured.
The structure shows that the IBR possesses two zinc-binding sites that adopt a
dual scissor-like and GAG knuckle-like fold. Furthermore, zinc binding is required
for the correct folding of the domain as substitution of zinc coordinating residues
(C332S, C365S) causes its global unfolding. The missense T351P mutation
found in patients suffering from Autosomal Recessive Juvenile Parkinson’s
(ref6a) causes the global unfolding of the IBR domain, whereas the mutation
R334C (ref7a) causes some structural rearrangement. In contrast, the protein
containing the mutation G328E appears to be properly folded.
Parkin IBR Domain Zinc
Coordinating Sites
Zinc
Coordinating
Site I
Zinc is essential for proper
folding of the IBR domain.
Any mutations
(deletions/substitutions) of
the key residues can
abolish zinc coordination
Zinc
Coordinating
Site II
and result in a dysfunctional
Parkin protein.
Zinc-Coordinating Residues Site I
Zn2+
Cysteine
Zinc-Coordinating Residues Site II
Zn2+
Histidine
Cysteine
Role of Parkin in Protein Degradation: A Link Between Loss-ofFunction and Juvenile Parkinsonism
The protein Parkin has been identified as an E3 ubiquitin–protein ligase
of the ubiquitin-proteosome system that is required to maintain cellular
protein quality control by removing misfolded or damaged proteins.
Parkin is involved in protein degradation as a ubiquitin-protein ligase
collaborating with the ubiquitin-conjugating protein Ubch7. Mutant parkin
from patients with autosomal recessive juvenile parkinsonism shows
loss of the ubiquitin-protein ligase activity. These findings indicated that
accumulation of proteins causes a selective neural cell death without
formation of Lewy bodies, which are absent in juvenile Parkinson’s.
The Link Between Parkin and Alpha-Synuclein, Another Protein
Involved in Parkinson’s Disease
It has been hypothesized that alpha-synuclein and parkin interact
functionally and that parkin normally ubiquitinates alpha-synuclein and that
this process is altered in PDJ. In contrast to normal parkin, mutant parkin
associated with autosomal recessive Parkinson disease failed to bind alphaSp22. Consequently, alpha-Sp22 accumulates in a nonubiquitinated form in
parkin-deficient Parkinson disease brains. These findings demonstrated a
critical biochemical reaction between the two Parkinson disease-linked gene
products and suggested that this reaction underlies the accumulation of
ubiquitinated alpha-synuclein in conventional Parkinson disease. [ref8a]
Interactions Between Parkin and Leucine-Rich Repeat Kinase 2
It has also been reported that parkin interacts with LRRK2. LRRK2
interacted preferentially with the C-terminal R2 RING finger domain of
parkin, and parkin interacted with the COR domain of LRRK2.
Works Cited
[1] Proc Natl Acad Sci U S A. 2008 Feb 5;105(5):1499-504. Epub 2008 Jan 29.
Structure of the ROC domain from the Parkinson's disease-associated leucine-rich repeat kinase 2 reveals a dimeric GTPase.
Deng J, Lewis PA, Greggio E, Sluch E, Beilina A, Cookson MR.
Department of Biochemistry and Molecular Biology, Oklahoma State University
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