[1, 2]. Being scaffolding proteins, Scribble, Dlg, and Lgl interact
with many other proteins using their specific domains, not only to
impart their functions in cell polarity but also to regulate cell
signalling, all leading to effects on cell proliferation, differentiation
and cellular migration [3].
Scribble belongs to the LAP (LRR and PDZ) protein family,
and consists of 16 Leucine-rich repeats (LRRs), two LAP specific
domains (LAPSADa and LAPSADb) and four PDZ (post-synaptic
density protein 95 (PSD 95), Discs large homologue (Dlg1) and
Zonula Occludens-1 (ZO1) domains [3]. Dlg belongs to the
membrane-associated guanylate kinase homolog (MAGUK) protein family, comprises a Src homology 3 (SH3) domain, a Hook
domain, three PDZ domains, and a guanylate kinase-like (GUK)
domain [3]. The majority of interactions of Scribble and Dlg are
mediated by their PDZ binding domains [3]. PDZ domains usually
comprise 80–100 amino acids forming two α-helices wrapping
around six β-strands with a single binding site between α-helix
2 and β-strand 2 [4, 5]. PDZ domains are associated with varying
biological processes such as target signaling, membrane protein
localization, cytoskeletal regulation, and cell polarity processes
[5]. They are highly conserved in structure and function throughout evolution and notably abundant in vertebrates, with over
250 PDZ domains in humans alone [4, 6, 7].
The combined use of phage display, protein microarray, yeast
two-hybrid, proteomics and structural approaches have delivered
significant insight into the mode of interaction of PDZ domains
with their interactors [5, 8–10]. PDZ domains typically target short
carboxyl terminal amino acid sequences of 7–10 residues in length
[11], where the last residue of the target C-terminal sequence is
designated as residue 0 and the preceding residues are counted
backward with the overall pattern being . . .(-3)-(-2)-(-1)-(0) COOH
. Three classes of PDZ domain binding motifs (PBMs) can be
distinguished: the first class consists of (X)-[T/S]-(X)-(ϕ) COOH
consensus sequence where X is any residue, and ϕ is any hydrophobic residue. The second PBM class comprises (X)-(ϕ)-(X)-(ϕ) COOH
and PBM class 3 (X)-[E/D])-(X)-(ϕ) COOH . PDZ domains can
target PBMs with varying selectivity and specificity depending on
the type of class a PBM contains [4, 5, 11]. In addition to
C-terminal PBMs, internal binding sites have also been identified
[12] including within the internal protein sequence of cell polarity
interactors [13, 14]. Additional modifiers of interactions include
post translational modifications such as phosphorylation, which
have been shown to be potent modifiers of binding affinities to
PDZ domains and can either decrease or enhance affinity [15–
17]. Dysregulation of PDZ domain mediated interactions by cell
polarity regulatory proteins is implicated in various disease such as
neurological and psychiatric disorders, viral infection, and cancer
[3, 18, 19]. Consequently an increased understanding of the
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Janesha C. Maddumage et al.
with many other proteins using their specific domains, not only to
impart their functions in cell polarity but also to regulate cell
signalling, all leading to effects on cell proliferation, differentiation
and cellular migration [3].
Scribble belongs to the LAP (LRR and PDZ) protein family,
and consists of 16 Leucine-rich repeats (LRRs), two LAP specific
domains (LAPSADa and LAPSADb) and four PDZ (post-synaptic
density protein 95 (PSD 95), Discs large homologue (Dlg1) and
Zonula Occludens-1 (ZO1) domains [3]. Dlg belongs to the
membrane-associated guanylate kinase homolog (MAGUK) protein family, comprises a Src homology 3 (SH3) domain, a Hook
domain, three PDZ domains, and a guanylate kinase-like (GUK)
domain [3]. The majority of interactions of Scribble and Dlg are
mediated by their PDZ binding domains [3]. PDZ domains usually
comprise 80–100 amino acids forming two α-helices wrapping
around six β-strands with a single binding site between α-helix
2 and β-strand 2 [4, 5]. PDZ domains are associated with varying
biological processes such as target signaling, membrane protein
localization, cytoskeletal regulation, and cell polarity processes
[5]. They are highly conserved in structure and function throughout evolution and notably abundant in vertebrates, with over
250 PDZ domains in humans alone [4, 6, 7].
The combined use of phage display, protein microarray, yeast
two-hybrid, proteomics and structural approaches have delivered
significant insight into the mode of interaction of PDZ domains
with their interactors [5, 8–10]. PDZ domains typically target short
carboxyl terminal amino acid sequences of 7–10 residues in length
[11], where the last residue of the target C-terminal sequence is
designated as residue 0 and the preceding residues are counted
backward with the overall pattern being . . .(-3)-(-2)-(-1)-(0) COOH
. Three classes of PDZ domain binding motifs (PBMs) can be
distinguished: the first class consists of (X)-[T/S]-(X)-(ϕ) COOH
consensus sequence where X is any residue, and ϕ is any hydrophobic residue. The second PBM class comprises (X)-(ϕ)-(X)-(ϕ) COOH
and PBM class 3 (X)-[E/D])-(X)-(ϕ) COOH . PDZ domains can
target PBMs with varying selectivity and specificity depending on
the type of class a PBM contains [4, 5, 11]. In addition to
C-terminal PBMs, internal binding sites have also been identified
[12] including within the internal protein sequence of cell polarity
interactors [13, 14]. Additional modifiers of interactions include
post translational modifications such as phosphorylation, which
have been shown to be potent modifiers of binding affinities to
PDZ domains and can either decrease or enhance affinity [15–
17]. Dysregulation of PDZ domain mediated interactions by cell
polarity regulatory proteins is implicated in various disease such as
neurological and psychiatric disorders, viral infection, and cancer
[3, 18, 19]. Consequently an increased understanding of the
126
Janesha C. Maddumage et al.