Figure 7: Model for the autoinhibition and activation of SAD-A.

(a) Sequence alignment for the AIS-KA1 regions of SAD and MARK kinases. The positively charged residues required for phospholipid binding are highlighted in blue. h, human; m, mouse; sp, Schizosaccharomyces pombe; sc, Saccharomyces cerevisiae. (b) Three basic clusters in the AIS-KA1 fragment. The basic residues are shown as blue sticks, and the surface representation is coloured according to electrostatic potential (positive, blue; negative, red). (c) Lipid-binding assays for two representative C-terminal fragments. PG, phosphatidylglycerol; PE, phosphatidylethanolamine; Chl, cholesterol; SPM, sphingomyelin; PS, phosphatidylserine; PIP₂, phosphatidylinositol-4,5-bisphosphate; PA, phosphatidic acid; PC, phosphatidylcholine. (d) Conserved (i–iv) and specific (SAD) hydrophobic pockets surrounding helix αC. Three AIS-binding pockets are indicated by green circles, and that for UBA binding is coloured in blue. For clarity, only helices α1 and α3 of UBA are displayed. (e) Regulation model for SAD-A. The extensive intramolecular interactions keep SAD-A in an autoinhibited conformation, where the kinase activity is synergistically inhibited by UBA and AIS. Membrane association of the AIS-KA1 fragment, probably triggered by regulatory protein(s), might release the AIS (and UBA) inhibition.