Descriptions

Mob1 is an activator of large tumor suppressor 1/2 (LATS1/2) in the Hippo signaling pathway which plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. Mob1b adopts an autoinhibited, closed conformation, with its N-terminal region. Truncation of N-terminal region (1-32), which removes most of the Switch helix, enhances binding of Mob1b to the NTR domain of LATS1. Phosphomimetic substitution of Thr12 and Thr35 with an aspartate residue also induces enhancement of LATS1 binding. Specifically, the N-terminal extension comprising a positively-charged region followed by Switch α-helix (20-39) binds directly to the LATS1-binding surface to block LATS1 binding and this blocking is stabilized by β-sheet formation between SN strand (5-9) of the N-terminal extension and S2 strand (94-98) of the core domain. The H1 helix of the autoinhibited form is followed by a sharp turn of Ser38-Gly39 and the Switch helix. The turn is stabilized by the Ser38 side chain forming hydrogen bonds with the main chains of Thr35 from the H1 helix and Asn40 from the Switch helix. This turn is absent in the phosphorylated form and the peptide chain is extended to run through the shallow groove formed by helices H6-H7 and H9 and is followed by a disordered region. The extended structure is stabilized by Ser38 interacting with the phosphate group of Thr35 and a short 310-helix (H0) formed by Leu-Leu-Lys at positions 28–30, which bind the hydrophobic site of the shallow groove. In the autoinhibited form, the Leu-Leu-Lys segment is part of the Switch helix and two Leu residues participate in nonpolar interactions with the H2 helix. The phosphorylated Thr12 residue of the pMOB1A-LATS1 complex is located close to the S2 strand so that the phosphate group is docked into the basic cluster pocket. The N-terminal end residues (1-9) are disordered but the three residues (13-15) that follow pThr12 form the S0 strand, which associates with the S2 strand to form a short antiparallel β-sheet. This S0-S2 β-sheet formation differs from that of the SN-S2 β-sheet found in the autoinhibited form in several ways. First, the SN strand of the autoinhibited form is formed by five residues and is longer than the S0 strand. Second, the residues forming the SN strand are located at positions 5-9 (Phe5 to Ser9) and represent a 6-residue shift toward the N-terminal end. It is likely that on phosphorylation, the peptide chain of the N-terminal extension is pulled down toward the basic cluster pocket for phosphorylated Thr12 binding, resulting in a frame shift of residues forming the β-strand associated with the S2 strand. This shift should induce dissociation of the Switch helix, which is also accelerated by phosphorylation of Thr35 located at the helix. This mechanism is called as the pull-the-string mechanism for MOB1 activation.

Accessory elements