Stepwise assembly of hyperaggregated forms of Drosophila zeste mutant protein suppresses white gene expression in vivo.

The zeste gene is involved in two chromosome pairing‐dependent phenomena: transvection and the suppression of white gene expression. Both require the ability of zeste protein to multimerize, dependent on three interlaced hydrophobic heptad repeats in the C‐terminal domain. The first step is dimerization through a leucine zipper. Two other heptad repeats are then required to form higher multimers. The zeta(1) mutation, which causes the pairing‐dependent suppression of white, creates a new hydrophobic nucleus that allows the formation of a new and larger aggregate. The zeta(op6) mutation, which suppresses even unpaired copies of white, makes even larger aggregates. The phenotypic suppression of white by a series of mutants is strictly correlated with hyperaggregation and the larger the hyperaggregates, the weaker the requirement for the pairing of white. Hyperaggregation of the Z1 protein in vitro is suppressed by co‐translation with the C‐terminal peptide of wild‐type protein, lacking the DNA‐binding domain. This C‐Z+ peptide also complements the zeta(1) allele in vivo and restores normal color, demonstrating that zeste product also exists in a multimeric complex in the cell. Complementation in vivo is strictly correlated with the prevention of hyperaggregation of the zeste mutant products in vitro, supporting the interpretation that excessive association of zeta(1) and zeta(op6) proteins is responsible for their repression of white gene expression.