SINGLE AMINO-ACID SUBSTITUTIONS IN EGF-LIKE ELEMENTS OF NOTCH AND DELTA-MODIFY DROSOPHILA DEVELOPMENT AND AFFECT CELL-ADHESION INVITRO

Notch locus EGF-like element mutations spl, altering eye development, and Ax(E2), affecting wing and sensilla development, are modified by mutations at Delta. It is shown that two allele-specific suppressors of spl involve single amino acid substitutions in the 4th (Dl(sup5)) and 9th (Dl(sup4)) EGF-like elements of the Delta protein. Cultured cells producing spl or Ax(E2) aggregate with cells producing wild-type Delta or Dl(sup5) protein, and Dl(sup5)-producing cells adhere to cells producing wild-type Notch protein. However, spl, Ax(E2), and Dl(sup5) are each defective in promoting these cell affinities, as none of the mutant proteins can compete with the corresponding wild-type proteins for formation of cell aggregates. Thus, widely separated EGF-like elements of Notch and Delta appear to participate in functional molecular interactions between the proteins. Dl(sup5) does not improve adhesiveness of spl in vitro, so suppression in vivo may involve altered developmental signaling by spl-Dl(sup5) complexes, rather than modified cell adhesion.