Structure
Volume 7, Issue 4, 15 April 1999, Pages 449-460
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Research Article
Structure of the PH domain from Bruton's tyrosine kinase in complex with inositol 1,3,4,5-tetrakisphosphate

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Abstract

Background: The activity of Bruton's tyrosine kinase (Btk) is important for the maturation of B cells. A variety of point mutations in this enzyme result in a severe human immunodeficiency known as X-linked agammaglobulinemia (XLA). Btk contains a pleckstrin-homology (PH) domain that specifically binds phosphatidylinositol 3,4,5-trisphosphate and, hence, responds to signalling via phosphatidylinositol 3-kinase. Point mutations in the PH domain might abolish membrane binding, preventing signalling via Btk.

Results: We have determined the crystal structures of the wild-type PH domain and a gain-of-function mutant E41K in complex with D-myo-inositol 1,3,4,5-tetra-kisphosphate (Ins (1,3,4,5)P4). The inositol Ins (1,3,4,5)P4 binds to a site that is similar to the inositol 1,4,5-trisphosphate binding site in the PH domain of phospholipase C-δ. A second Ins (1,3,4,5)P4 molecule is associated with the domain of the E41K mutant, suggesting a mechanism for its constitutive interaction with membrane. The affinities of Ins (1,3,4,5)P4 to the wild type (Kd = 40 nM), and several XLA-causing mutants have been measured using isothermal titration calorimetry.

Conclusions: Our data provide an explanation for the specificity and high affinity of the interaction with phosphatidylinositol 3,4,5-trisphosphate and lead to a classification of the XLA mutations that reside in the Btk PH domain. Mis-sense mutations that do not simply destabilize the PH fold either directly affect the interaction with the phosphates of the lipid head group or change electrostatic properties of the lipid-binding site. One point mutation (Q127H) cannot be explained by these facts, suggesting that the PH domain of Btk carries an additional function such as interaction with a Gα protein.

Keywords

crystal structure
human
immunodeficiency
inositol phosphates
PH domain
tyrosine kinase

Cited by (0)

E Baraldi, KD Carugo, M Hyvönen and PL Surdo, European Molecular Biology Laboratory, Meyerhofstrasse 1, Postfach 102209, D-69012, Heidelberg, Germany.

Present address for M Hyvönen, Department of Biochemistry, University of Cambridge, 80 Tennis Court Road, Cambridge CB2 1GA, UK.

AM Riley, BV Potter and M Saraste, Wolfson Laboratory of Medicinal Chemistry, Department of Pharmacy and Pharmacology, University of Bath, Claverton Down, Bath BA2 7AY, UK.

R O’Brien and JE Ladbury, Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, UK.

E-mail address for M Saraste (corresponding author): [email protected].