SH3 DOMAINS SPECIFICALLY REGULATE KINASE-ACTIVITY OF EXPRESSED SRC FAMILY PROTEINS

The Src homology 2 (SH2) and Src homology 3 (SH3) domain are approxima tely 50% conserved in various Src family kinase members. Several lines of evidence suggest that in Src these domains are sequence motifs tha t direct substrate recognition, regulate kinase activity, or control s ubcellular localization, We sought to investigate the function of the homology domains in human Lyn, and to determine whether the difference s between various SH3 domains affect function. To do this, we generate d variant forms of Lyn lacking SH2 and SH3 domains, and created chimer as in which the SH3 domains in human c-Src and Lyn were replaced with SH3 domains from other family members. In contrast to similar deletion s in Src, forms of Lyn lacking SH2 or SH3 had decreased kinase activit y. The SH3 chimeras all had individual characteristics. Insertion of t he Blk SH3 domain into Lyn restored kinase activity, while insertion o f the Fyn or Src SH3 into Lyn enhanced the kinase activity 2-3-fold. I nsertion of the Lyn SH3 into Src also doubled kinase activity, Express ion of the Lyn-Src SH3 chimera in mammalian cells induced cell transfo rmation, This study 1) demonstrates that the regulation of Lyn is diff erent than Src, and 2) provides new evidence that despite their homolo gy, there are important functional differences between the SH3 domains of the various Src family members.