view of how the numerous environmen

view of how the numerous environmental or hormonal signals that
intervene during plant growth and development target, in time and
space, the multiple aquaporin isoforms expressed throughout the plant.
4.2. Gating
The gating of plant aquaporins, that is, the opening and closing of the
water channel pore, can be regulated by multiple effectors, such as
phosphorylation, protons (H+), and divalent cations. An activating
role for phosphorylation was first proposed for pea PvTIP3;1,
GmNod26 and spinach SoPIP2;1, based on functional expression in
oocytes of wild-type and phosphorylation mutant forms, together
with alterations of oocyte protein phosphatase and/or kinase activities
[59,63,69]. Water transport assays in purified plasma membranes
from Arabidopsis and sugar beet (Beta vulgaris) also indicated that H+
and divalent cations, with calcium (Ca2+) being the most efficient, can
regulate the activity of PIPs [70,71]. These effects were further
established after reconstitution of purified AtPIP2;1 in proteoliposomes
[72]. Furthermore, the sidedness and molecular bases of H+-dependent
gating were dissected using Xenopus oocyte expression and the central
role as pH sensor of a His residue, perfectly conserved in the second
cytosolic loop (loop D) of PIPs, was established [55] (Fig. 1).
The X-ray structures of SoPIP2;1 in an open and closed conformation
[33] were crucial in determining a conserved molecular mechanism for
PIP gating. In brief, the model showed how a hydrophobic (Leu) residue
of loop D can protrude in the cytosolic pore vestibule, to prevent any
passage of water. The conformation of loop D itself is determined by
ionic and H-bond interactions between residues of loops D (His193)
and B (Ser115) and the N-terminal tail (Asp28, Glu31). Divalent cations
binding to Asp28 and Glu31 or protonation of loop D at His193
can tighten this interaction, locking the water channel in a closed
state. By contrast, phosphorylation of Ser115 in loop B or Ser274 in
the C-terminus favors the unfolding of loop D to open the water
channel.

ผลลัพธ์ (ไทย) 3:

ไม่แก่หรอกไม่แก่หรอกไม่แก่หรอกไม่แก่หรอกไม่แก่หรอกไม่แก่หรอก