Mad1s ability to interact with Mad2 is essential to regulate and monitor meiotic synapsis in C. elegans

Abstract

Meiotic homolog synapsis is essential to ensure accurate segregation of chromosomes during meiosis. In C. elegans, synapsis and a checkpoint that monitors synapsis relies on the spindle checkpoint components, Mad1 and Mad2, and Pairing Centers (PCs), cis-acting loci that interact with the nuclear envelope to mobilize chromosomes within the nucleus. Here, we show that mutations in some spindle checkpoint mutants affect PC movement early in meiotic prophase, consistent with a link between PC mobility and the regulation of synapsis. Further, we test what specific functions of Mad1 and Mad2 are required to regulate and monitor synapsis. We find that a mutation that abrogates Mad1s localization to the nuclear periphery abolishes the synapsis checkpoint but has no effect on Mad2s localization to the nuclear periphery or synapsis. By contrast, a mutation that prevents Mad1s interaction with Mad2 abolishes the synapsis checkpoint, delays synapsis and fails to localize Mad2 to the nuclear periphery. These data indicate that Mad1s primary role in regulating synapsis is through control of Mad2 and that Mad2 can bind other factors at the nuclear periphery. We also tested whether Mad2s ability to adopt a specific conformation associated with its activity during spindle checkpoint function is required for its role in meiosis. A mutation that prevents Mad2 from adopting its active conformer fails to localize to the nuclear periphery, abolishes the synapsis checkpoint and exhibits substantial defects in meiotic synapsis. Thus, Mad2, and its regulation by Mad1, is a major regulator of meiotic synapsis in C. elegans. AUTHOR SUMMARYSexual reproduction relies on production of gametes, such as eggs and sperm, which are produced during meiosis. During this specialized cell division, chromosomes replicate, pair with their homologs, undergo synapsis and finally undergo recombination, all of which are required for correct meiotic chromosome segregation. Chromosomes are highly mobile during these steps in meiosis but the specific role of this mobility is unclear. Here, we show that spindle assembly checkpoint proteins, Mad1 and Bub3, that regulate and monitor meiotic synapsis are implicated in chromosome movement, solidifying the functional link between chromosome mobility and synapsis. Moreover, we provide additional data that another spindle checkpoint effector, Mad2, and its regulation by Mad1, plays an important role in regulating meiotic synapsis.