A Physiological, Biochemical and Structural Analysis of Inositol Polyphosphate 5-Phosphatases from Arabidopsis thaliana and Humans
The complete role of inositol signaling in plants and humans is still elusive. The plant Arabidopsis thaliana contains fifteen predicted inositol polyphosphate 5- phosphatases (5PTases, E.C. 188.8.131.52) that have the potential to remove a 5-phosphate from various inositol second messenger substrates. To examine the substrate specificity of one of these Arabidopsis thaliana 5PTases (At5PTases), recombinant At5PTase1 was obtained from a Drosophila melanogaster expression system and analyzed biochemically. This analysis revealed that At5PTase1 has the ability to catalyze the hydrolysis of four potential inositol second messenger substrates.
To determine whether At5PTase1 can be used to alter the signal transduction pathway of the major drought-sensing hormone abscisic acid (ABA), plants ectopically expressing At5PTase1 under the control of a constitutive promoter were characterized. This characterization revealed that plants ectopically expressing At5PTase1 had an altered response to ABA. These plants have stomata that are insensitive to ABA, and have lower basal and ABA-induced inositol (1,4,5)-trisphosphate [Ins(1,4,5)P₃] levels. In addition, At5PTase1 mRNA and protein levels are transiently regulated by ABA. These data strongly suggest that At5PTase1 can act as a signal terminator of ABA signal transduction.
Like the Arabidopsis At5PTase1, a human 5PTase, Ocrl, has the ability to catalyze the hydrolysis of a 5-phosphate from several inositol-containing substrates. The loss of functional Ocrl protein results in a rare genetic disorder known as Lowe oculocerebrorenal syndrome. To gather information concerning the specificity determinants of the Ocrl protein, a structure-function analysis of Ocrl was conducted using a vibrational technique, difference Fourier transform infrared (FT-IR) spectroscopy. Upon the introduction of Ins(1,4,5)P₃ substrate, structural changes in carboxylic acid and histidine residues were observed. The net result of changes in these residues indicates that upon Ins(1,4,5)P₃ introduction, a carboxylic acid-containing residue is protonated, and a histidine residue is deprotonated. This interpretation supports the idea that the deprotonation of the histidine residue is concomitant with the coordination of a divalent cation upon Ins(1,4,5)P₃ introduction. This work allows for the proposal of a new model for the role of the active site histidine of OCRL.