interaction with DpbA and Occludin. By applying linearly increasing forces to purified full-length single ZO1 molecules with magnetic tweezers, Spadaro et al. could demonstrate that both the
C-terminal region and the ZPSG-1 module of ZO1 that comprises
the PDZ3, SH3, U5, and GUK domains unfold at forces ranging
from 5 to 20 pN, releasing the autoinhibition interaction between
ZPSG and C terminal domain of ZO1 [31]. ZPSG-1 domain is not
only important for the junctional localization of ZO1 but also for
its interaction with Occludin, DbpA, and thus for barrier formation
and epithelial polarization [33–38]. Forces act as an allosteric effector by stretching ZO1 protein to promote the interaction of ZPSG1 with its ligands, occludin and DbpA. While the stretched conformation of ZO1 is the active conformation, the folded conformation
is the inactive form in which the ZPSG domain is autoinhibited. In
its inactive form, junctional ZO1 remains anchored to the membrane through binding of its N-terminal domain with interactors
such as TAZ or Claudins, while the C-terminal half is intramolecularly autoinhibited. The release of ZPSG-1 domains may also regulate the interaction of other proteins such as α-Catenin, Afadin,
JAM-A, Vinculin and Shroom2 [39].
3.2 MUPP1
MUPP1 belongs to the family of multi-PDZ proteins and contains
a L27 domain at its N-terminal region followed by 13 PDZ
domains [40]. MUPP1 is a structural paralog of PATJ, and both
share several binding partners such as PALS1, PAR-6, AMOT,
Jeap, ZO3, Claudins, or Nectins [41]. Lanaspa et al. by the use of
osmolarity changes demonstrated that both acute and chronic
hyperosmolarity in inner medullary collecting duct 3 cells induce
an increase in the expression of MUPP1, ZO1, and Afadin [42]. As
MUPP1 expression increases, it localizes to the apical side of the
membrane at the level of the Tight Junctions (TJs). To survive
hyperosmotic stresses and to maintain the integrity of the cell
sheet with efficient barrier functions, cells have to adapt through
ä
Fig. 1 (continued) conformation, and the second line mechanotransducers Afadin and Mupp1 are in the
cytoplasm. The actin filaments are exerting low mechanical loads. Under high tension, generated by
actomyosin bundles, ZO1 unfolds and unmasks several domains that leads to the binding with Occludin
and JAM together with the recruitment of DbpA and Afadin. Afadin is then able to recruit Rap2c generating a
positive feedback loop on RHO allowing for actin contractions and thus force generation. This increase in
forces results in the recruitment of Mupp1 at the TJs, that will bind to ZO1, ZO2, and JAM but also potentially
to the CRB3A/Pals1 polarity complex. The formation of this macromolecular complex that tethers transmembrane proteins (Claudins, Occludins, JAM) to the actin cytoskeleton is key for the strengthening and the
stabilization of the TJs. (b) In the lateral domain of the cells, at the level of the lateral Adherens Junctions
(LAJs), the actin filaments that binds to the E-cadherin–catenin complex, exert low tension and SCRIBBLE is in
the cytoplasm. Above the lateral junctions, the E-cadherin–catenin complex is link to vinculin and binds to
actomyosin bundles that are under high mechanical loads. SCRIBBLE is also enriched at the AJ level, and
plays a key role in the stabilization of the AJs
Mechanoregulation of PDZ Proteins
261
C-terminal region and the ZPSG-1 module of ZO1 that comprises
the PDZ3, SH3, U5, and GUK domains unfold at forces ranging
from 5 to 20 pN, releasing the autoinhibition interaction between
ZPSG and C terminal domain of ZO1 [31]. ZPSG-1 domain is not
only important for the junctional localization of ZO1 but also for
its interaction with Occludin, DbpA, and thus for barrier formation
and epithelial polarization [33–38]. Forces act as an allosteric effector by stretching ZO1 protein to promote the interaction of ZPSG1 with its ligands, occludin and DbpA. While the stretched conformation of ZO1 is the active conformation, the folded conformation
is the inactive form in which the ZPSG domain is autoinhibited. In
its inactive form, junctional ZO1 remains anchored to the membrane through binding of its N-terminal domain with interactors
such as TAZ or Claudins, while the C-terminal half is intramolecularly autoinhibited. The release of ZPSG-1 domains may also regulate the interaction of other proteins such as α-Catenin, Afadin,
JAM-A, Vinculin and Shroom2 [39].
3.2 MUPP1
MUPP1 belongs to the family of multi-PDZ proteins and contains
a L27 domain at its N-terminal region followed by 13 PDZ
domains [40]. MUPP1 is a structural paralog of PATJ, and both
share several binding partners such as PALS1, PAR-6, AMOT,
Jeap, ZO3, Claudins, or Nectins [41]. Lanaspa et al. by the use of
osmolarity changes demonstrated that both acute and chronic
hyperosmolarity in inner medullary collecting duct 3 cells induce
an increase in the expression of MUPP1, ZO1, and Afadin [42]. As
MUPP1 expression increases, it localizes to the apical side of the
membrane at the level of the Tight Junctions (TJs). To survive
hyperosmotic stresses and to maintain the integrity of the cell
sheet with efficient barrier functions, cells have to adapt through
ä
Fig. 1 (continued) conformation, and the second line mechanotransducers Afadin and Mupp1 are in the
cytoplasm. The actin filaments are exerting low mechanical loads. Under high tension, generated by
actomyosin bundles, ZO1 unfolds and unmasks several domains that leads to the binding with Occludin
and JAM together with the recruitment of DbpA and Afadin. Afadin is then able to recruit Rap2c generating a
positive feedback loop on RHO allowing for actin contractions and thus force generation. This increase in
forces results in the recruitment of Mupp1 at the TJs, that will bind to ZO1, ZO2, and JAM but also potentially
to the CRB3A/Pals1 polarity complex. The formation of this macromolecular complex that tethers transmembrane proteins (Claudins, Occludins, JAM) to the actin cytoskeleton is key for the strengthening and the
stabilization of the TJs. (b) In the lateral domain of the cells, at the level of the lateral Adherens Junctions
(LAJs), the actin filaments that binds to the E-cadherin–catenin complex, exert low tension and SCRIBBLE is in
the cytoplasm. Above the lateral junctions, the E-cadherin–catenin complex is link to vinculin and binds to
actomyosin bundles that are under high mechanical loads. SCRIBBLE is also enriched at the AJ level, and
plays a key role in the stabilization of the AJs
Mechanoregulation of PDZ Proteins
261