STEPWISE ASSEMBLY OF HYPERAGGREGATED FORMS OF DROSOPHILA-ZESTE MUTANTPROTEIN SUPPRESSES WHITE GENE-EXPRESSION INVIVO

The zeste gene is involved in two chromosome pairing-dependent phenome na: transvection and the suppression of white gene expression. Both re quire the ability of zeste protein to multimerize, dependent on three interlaced hydrophobic heptad repeats in the C-terminal domain. The fi rst step is dimerization through a leucine zipper. Two other heptad re peats are then required to form higher multimers. The z1 mutation, whi ch causes the pairing-dependent suppression of white, creates a new hy drophobic nucleus that allows the formation of a new and larger aggreg ate. The z(op6) mutation, which suppresses even unpaired copies of whi te, makes even larger aggregates. The phenotypic suppression of white by a series of mutants is strictly correlated with hyperaggregation an d the larger the hyperaggregates, the weaker the requirement for the p airing of white. Hyperaggregation of the Z1 protein in vitro is suppre ssed by co-translation with the C-terminal peptide of wild-type protei n, lacking the DNA-binding domain. This C-Z+ peptide also complements the z1 allele in vivo and restores normal eye color, demonstrating tha t zeste product also exists in a multimeric complex in the cell. Compl ementation in vivo is strictly correlated. with the prevention of hype raggregation of the zeste mutant products in vitro, supporting the int erpretation that excessive association of z1 and z(op6) proteins is re sponsible for their repression of white gene expression.