Saccharomyces cerevisiae KAR4 Regulates Multiple Meiotic Pathways

Abstract

In mating, Kar4p works with Ste12p to activate the transcription of genes required for karyogamy, but its function in meiosis is unknown. The purpose of this work is to further characterize the regulation and function of KAR4 in mating and meiosis. To determine whether Kar4p has a distinct function in meiosis, we designed a genetic screen for mutant alleles that might differentially affect its function in these two pathways. We find that Kar4p's functions are distinct and separable. Characterization of the KAR4 alleles that came out of the screen revealed that loss of mating function is due to disruption of the Kar4p-Ste12p protein-protein interaction, and that Kar4p has multiple meiotic functions.

Kar4p is required at least twice in meiosis in the S288C strain background, although it is dispensable in SK1. Cells without KAR4 fail to initiate meiotic DNA replication, although the key early meiotic regulatory genes IME1 and IME2 are induced. However, overexpression of IME1, IME2, or a third regulator, RIM4, can suppress the first kar4 meiotic phenotype. A subset of the mutant kar4 alleles go on to complete meiosis and form spores when IME1 overexpression bypasses the first requirement for Kar4p. However, other alleles (including some defective for mating) remained blocked at the pachytene checkpoint, suggesting that Kar4p has a second function in meiosis. Mapping of the alleles on a threaded structure of Kar4p suggests that distinct regions of the protein are required for mating and each of the two meiotic functions. Interestingly, combined overexpression of IME1 and RIM4 leads to spore formation in kar4 diploids, although overexpression of IME1 alone is sufficient only to drive recombination.

Kar4p also interacts with the meiotic mRNA adenine methyl-transferase Ime4p. However, mutations that disrupt this interaction have no effect on meiosis, suggesting that the interaction is neither necessary nor sufficient for either of Kar4p's meiotic functions. Finally, although Kar4p is phosphorylated at S329, this modification is not required for any of its known functions.

We conclude that KAR4 is a major regulator of at least two steps in the meiotic pathway, and that this role is independent from its function in mating.