Band 4.1 and ERM Proteins

1
Band 4.1 and ERM Proteins

• Weiwei Chen
• Michele Kadnar
• Kwame Osei-Sarfo
• Rebecca Wendland

2
Band 4.1 Superfamily

• Highly conserved domain FERM (4.1-ezrin, radixin,
  moesin)
• Based on cDNA/protein sequences
• Band 4.1 protein
• ERM (ezrin/radixin/moesin)
• Merlin (schwannomin)
• FERM-PTP
  (protein tyrosine phosphatase)
• NBLs (novel band 4.1-like proteins)
• Talin

Pearson et al., 2000
3
Band 4.1 ERM

• Sequence similarity compared to human ezrin

FERM ?-domain CTD
Human band 4.1
NH2-
32 8 15

ERMs

• Differences
• - Band 4.1, unique conserved CTD, function
  in nucleus
• less conserved SABD
  (spectrin/actin binding domain)
• - ERM, CTD interact with actin have some
  different membrane binding proteins
  intramolecular regulation
• Commonalities
• - structure, some binding partners, some
  functions

4
Band 4.1 family

• 4.1 R hematopoietic tissues, erythrocyte
• 4.1 G multiple tissues
• 4.1 N brain (neurons)
• 4.1 B brain
• Coracle Drosophila, C. elegans epithelial
  tissues

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Band 4.1 traditional functions
CaM and phosphorylation regulation - cell
morphological dynamic
Bennett V and Baines AJ, 2001
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Interacting proteins emerging functions
p55(MAGUK)
spectrin, actin
Band 3
Lobe 3
Lobe 1
PIP2
NuMA (nuclear mitotic apparatus protein), FKBP13
(immunophilin)
glycophorin C, neurexins, syndecan,
CD44
Lobe 2
calmodulin
human discs-large
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ERM Family of Proteins

• 4.1 superfamily
• Members
• Protein 4.1
• Ezrin, radixin, moesin
• Merlin/Schwannomin
• ERM identity
• gt80 NTD
• gt60 overall
• Merlin identity
• - 46 identity with ERMs

NTD
CTD
?-helix
FERM Domain
Louvet-Vallee, 2000
8
ERM Function

• Membrane Organization-Interaction with PM and
  actin cytoskeleton
• Attach actin filaments to PM maintaining cell
  shape
• Cell communication
• Cluster ion channels and receptors
• Membrane dynamics
• Form cell surface structures, i.e. membrane
  ruffles
• Involved in cell-adhesion
• Membrane trafficking
• Tumor suppression
• Merlin -neurofibramatosis tumor suppressor

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ERM Distribution

• In vivo tissue-specific pattern
• Ezrin- intestine, stomach, lung, kidney
  epithelial and mesothelial cells
• Moesin- lung, spleen endothelial cells
• Radixin- liver, intestine
• Merlin- heart, brain, lung, liver, skeletal
  muscle
• All concentrated in actin-rich surface
  structures, i.e. microvilli, membrane ruffles,
  filopodia
• Merlin mutation is tumorigenic
• ERMs functions are redundant in whole body
  system
• Moesin KO is not lethal, normal phenotype
• No implication in cell proliferation phenotype

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ERM Family Binding Partners

• N-term, via FERM domain, binds to the cytoplasmic
  face of several transmembrane glycoproteins
• C-term binds to actin filaments
• N-term highly conserved

FERM
CTD
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Direct vs Indirect Binding to TM Proteins

• NTD can bind directly to TM protein
• NTD has binding site for NHE-RF (EBP-50)
• NHF-RF is ion exchanger
• NHE-RF interacts with proteins via PDZ domain
• Widely distributed in polarized epithelia
• Indirect binding offers another level of
  regulation

Ezrin Moesin
NHE-RF (EBP-50)
Direct Binding
Indirect Binding
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Conformational dependence

• Actin-binding region resides in CTD tail
• Conformational dependence-
• Inactive-folded
• Active- extended
• CTD has extended peptide that masks the FERM
  domain
• CTD has residues that allow the FERM domain to
  complete folding

Pearson, 2000
13
Regulation

• Intracellular and extracellular signals
• Change from inactive to active conformation
• phosphorylation by EGF receptor of ERM
  serine/threonine residues weakens the CTD/FERM
  interaction
• PIP2 binds to NTD suppressing the interaction of
  CTD/FERM
• Extended (active) conformation is a novel
  mechanism for producing varying levels of
  activation

14
Rho signaling pathway

• Rho can regulate actin-based cytoskeletal
  organization
• activated by G-protein coupled receptors
• activates several serine/threonine kinases
• activates PI4P5K activating PIP2
• Positive feedback system

Louvet-Vallee, 2000