A, Alpha1 does not bind Dab1 in the embryonic brain. Proteins were extracted from wild type embryonic mouse brain and immunoprecipitated with Alpha1, Alpha2, Lis1 or control Myc antibodies and probed with Dab1 antibodies (IP). Dab1 coprecipitated only with Alpha2 or Lis1. B, Amino acid sequence alignment of a highly conserved N terminal region of mouse Alpha1 and Alpha2 proteins containing residue 56 (arrow). C, FLAG-tagged Lis1, GFP-tagged Alpha1 or Alpha2 and HA-tagged Dab1 were coexpressed in COS7 cells. Proteins were immunoprecipitated with the indicated antibodies and probed with GFP antibodies to detect Alpha1 or Alpha2 (IP). The lysates were also directly probed with GFP antibodies to ensure that Alpha1 or Alpha2 proteins were expressed at similar levels. Mutant Alpha2(Y56C), like Alpha1, cannot bind Dab1.