1995) Nature 386, 527-530). In this paper we describe Gap1 m , which is closely related to Gap1 IP4BP , to also be an IP 4 -binding protein and show that the pleckstrin homology domain (PH) is the central IP 4 -binding domain by expressing fragments of the mouse Gap1 m in Escherichia coli as fusion proteins and examining their activities. However, in addition to the PH domain, an adjacent GAP-related domain and carboxyl terminus are required for high affinity specific IP 4 binding. The PH

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... is highly conserved in the Gap1 family and also has striking homology to the amino-terminal region of Bruton's tyrosine kinase. Substitution of Cys for Arg at position 628 in the PH domain corresponding to the mutation of Bruton's tyrosine kinase observed in X-linked immunodeficiency mice results in a dramatic reduction of IP 4 binding activity as well as phospholipid binding capacity of Gap1 m . This mutant also showed the GAP activity against Ha-Ras to be similar to that of the wild type Gap1 m . Our results suggest that the PH domain of Gap1 m functions as a modulatory domain of GAP activity by binding IP 4 and phospholipids.