Title : Structure function and regulation of the plasma membrane NaH antiporter Salt Overly Sensitive 1 SOS1 in plants
Abstract:
Short Description of what will be discussed during the presentation (about 250 - 500 words)
Physiological studies have confirmed that export of Na+ to improve salt tolerance in plants is regulated by the combined activities of a complex transport system. In the Na+ transport system, a plasma membrane Na+/H+ antiporter salt overly sensitive 1 (SOS1) is the main protein that functions to excrete Na+ out of plant cells. While in roots exposed to increased salinity, the activated protein kinase complex composed of CBL4 (also known as SOS3) and CIPK24 (also known as SOS2) activates SOS1, in shoots CBL10 interacts with and phosphorylates CIPK24, and the complex then promotes Na+ extrusion mediated by SOS1. Furthermore, CIPK8, the closest homolog to CIPK24 in the CIPK family, interacts with CBL10, and the CBL10-CIPK8 complex can also activate SOS1 activity in the shoots. Another homolog of CBL4, CBL5 is mainly expressed in the cotyledons and hypocotyls, and functions in seed germination and protects seeds and germinating seedlings from salt stress through the CBL5?CIPK8/CIPK24?SOS1 pathways. Therefore, these SOS signalling pathways, CBL4/5/10-CIPK8/24-SOS1, are the paramount regulatory mechanisms for facilitating Na+ extrusion and are critical to the ability of plants to adapt to and tolerate conditions of increased soil salinity.
SOS1 proteins from many plant spp. have been identified, and their structures are highly conserved and consisted of a transmembrane N?terminus and an extended C?terminal region involved in enzyme regulation. Without salt stress, the Na+/H+ exchange activity of AtSOS1 is inhibited because of the intramolecular interaction between the auto-inhibitory domain and activation domain in its C-terminal region. Upon exposure to salinity, the Ca2+ concentration increases in the cytosol. CBL4 responds to the changes in the calcium signal triggered by salt stress, and binds to Ca2+ and then interacts with CIPK24. The protein kinase complex phosphorylates a serine residue at the end of the auto-inhibitory domain and activates Na+/H+ exchange activity of SOS1. Furthermore, the other serine residue, which is at the beginning of the auto-inhibitory domain, is a regulation site of CBL4-CIPK24 complex? too. Token together, the two regulatory sites are at two ends of the inhibitory domain, respectively, and the regulatory domain overlaps with the inhibitory domain in the C-terminal, such that these two regulatory ways may function coordinately to more efficiently regulate SOS1 activity when plants encounter salt stress. These findings suggest that the auto-inhibitory domain completely hamper SOS1 activity, so its deletion can fully activate Na+ extrusion mediated by SOS1, and the overexpression of the modified gene confers the stronger salt tolerance in transgenic plants. Therefore, the hyperactive mutant of SOS1 may be a valuable tool for developing salt-tolerant transgenic crops.
