ACTIVATION OF BRUTONS TYROSINE KINASE (BTK) BY A POINT MUTATION IN ITS PLECKSTRIN HOMOLOGY (PH) DOMAIN

Bruton's tyrosine kinase (BTK) is a nonreceptor tyrosine kinase critic al for B cell development and function. Mutations in BTK result in X-l inked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (rid) in mice. Using a random mutagenesis scheme, we isolated a gain- of-function mutant called BTK whose expression drives growth of NIH 3 T3 cells in soft agar. BTK results from a single point mutation in th e pleck-strin homology (PH) domain, where a Glu is replaced by Lys at residue 41. BTK shows an increase in phosphorylation on tyrosine resi dues and an increase in membrane targeting. Transforming activity requ ires kinase activity, a putative autophosphorylation site, and a funct ional PH domain. Mutation of the SH2 or SH3 domains did not affect the activity of BTK. Expression of BTK* could also relieve IL-5 dependen ce of a a lineage cell line. These results show that transformation ac tivation and regulation of BTK are critically dependent on the PH doma in.