Architecture and intrasteric regulation of giant protein kinases

Abstract

C-185cellular signals into intracellular responses. Upon nucleotide exchange catalyzed by activated receptors, heterotrimers dissociate into Ga-GTP subunits and Gsv dimers. either of which can modulate numerous downstream effectors. The crystal structures of the trimeric (Gra.py). dimeric CGtpy). and three monomeric forms (Gw:·GDP, Gw·GTPyS. Gw·GDP·AIF...-) of transducin, the heterotrimeric G protein involved in the visual pathway, have been solved and refined to high resolution.The heterotrimeric form of transducin, Gmi'Y• reveals the mechanism of the nucleotide dependent engagement/disengagement between the a and ~y subunits that regulates their interaction with receptor and effector molecules. The interaction involves two distinct interfaces and dramatically alters the conformation of the a but not the ~y subunits. The location of the known sites for post-translational modification and receptor coupling suggest a plausible orientation for the heterotrimer with respect to both the membrane smface and the activated heptahelical receptor.Multiwavelength anomalous diffraction data were used to solve the crystal structure of Gtpy . The ~ subunit is primarily a seven-bladed ~-propeller that is partially encircled by an extended y subunit. The ~-propeller, which contains seven structurally similar WD-repeats, defines the stereochemistry of the WD-repeat and the probable architecture of all \VD-repeat containing domains. The structure details interactions between G protein ~ andy subunits and highlights regions implicated in effector modulation for the conserved family of G protein ~y dimers. Like most of the members of the receptor-like protein tyrosine phosphatase (RPTP) family, the protein tyrosine phosphatase alpha (PTPa) is involved in key cellular functions such as cell growth division and differentiation. All exhibit common features: an extracellular N-terminal domain, a membrane-spanning helix and one or two intracellular domains (D l and D2) displaying tyrosine phosphatase activity at different levels. Extracellular domains vary drastically in structure and are likely to bind ligands. Here we describe the X-ray structure of PTPa's domain Dl. An unexpected tight. symmetric and apparently inactive dimer was present in the crystal. The amino-terminal segment of one monomer which adopts a helix-loop-helix conformation is wedged into the other monomer's active site preventing the swing of its catalytically essential loop. This amino-terminal motif resembles an elbow and the encompassed sequence is conserved amongst D 1 domains of all the receptor-like PTPs (except PTPx) and not in D2 domains or non-receptor PTPs. We propose that dimerization of D l domains is a physiological event inducing a reversible inactive state of the enzyme as a consequence of colocalization of the intracellular domains by extracellular mediated events. In efforts to understand intermolecular interactions that control signal transduction, we have studied the structure and function of pleckstrin homology (PH) domains. These domains of= 120 ...