Absence of SUN-domain protein Slp1 blocks karyogamy and switches meiotic recombination and synapsis from homologs to sister chromatids

Abstract

Karyogamy, the process of nuclear fusion is required for two haploid gamete nuclei to form a zygote. Also, in haplobiontic organisms, karyogamy is required to produce the diploid nucleus/cell that then enters meiosis. We identify sun like protein 1 (Slp1), member of the mid-Sad1p, UNC-84-domain ubiquitous family, as essential for karyogamy in the filamentous fungus Sordaria macrospora, thus uncovering a new function for this protein family. Slp1 is required at the last step, nuclear fusion, not for earlier events including nuclear movements, recognition, and juxtaposition. Correspondingly, like other family members, Slp1 localizes to the endoplasmic reticulum and also to its extensions comprising the nuclear envelope. Remarkably, despite the absence of nuclear fusion in the slp1 null mutant, meiosis proceeds efficiently in the two haploid "twin" nuclei, by the same program and timing as in diploid nuclei with a single dramatic exception: the normal prophase program of recombination and synapsis between homologous chromosomes, including loading of recombination and synaptonemal complex proteins, occurs instead between sister chromatids. Moreover, the numbers of recombinationinitiating double-strand breaks (DSBs) and ensuing recombinational interactions, including foci of the essential crossover factor Homo sapiens enhancer of invasion 10 (Hei10), occur at half the diploid level in each haploid nucleus, implying per-chromosome specification of DSB formation. Further, the distribution of Hei10 foci shows interference like in diploid meiosis. Centromere and spindle dynamics, however, still occur in the diploid mode during the two meiotic divisions. These observations imply that the prophase program senses absence of karyogamy and/or absence of a homolog partner and adjusts the interchromosomal interaction program accordingly.sisters versus homologs | recombination/synapsis K aryogamy is the process by which two nuclei fuse to produce a single nucleus. This process is critical in diploid organisms (e.g., mammals and plants) when haploid egg and sperm nuclei fuse to produce a diploid nucleus and zygote. In organisms with a haploid vegetative cycle (e.g., fungi and most algae), karyogamy is required to produce the diploid nucleus/cell, which will then enter meiosis. In both situations, karyogamy involves two steps: nuclear movement leading to nuclear juxtaposition (congression) and final fusion of the nuclear membranes. Cooperation of the cytoskeleton components is required for nuclear movement and correct positioning of the nuclei (e.g., ref. 1).In budding yeast, premeiotic karyogamy requires several genes (named Kar1 to Kar9) with their mutant defects corresponding to two steps: nuclear congression, which involves cytoskeleton components, motor proteins and the spindle pole body (SPB, mammal centrosome equivalent) and fusion of the two haploid nuclear envelopes (NEs), which involves either the endoplasmic reticulum (ER) per se or protein translocation into the perinuclear space (reviewed in refs. 2-4 and references t...

Slp1-Emp65: A Guardian Factor that Protects Folding Polypeptides from Promiscuous Degradation

“…The genes encoding the Slp1-Emp65 complex are evolutionary conserved among all eukaryotes analyzed and make up a distinct subfamily of SUN domain family (Vasnier et al, 2014). In mammals, the Slp1 and Emp65 homologs are called SUCO (or OPT) and TAPT1, respectively.…”

Control of Meiotic Crossovers: From Double-Strand Break Formation to Designation

“…During mitosis, HEI10 is thought to cause degradation of cyclin B in the G2/M transition and also inhibits migration and invasion of cancer cells (203, 217). Other HEI10 orthologs have been identified in rice, A. thaliana , Sordaria , and a type of scallop (42, 135, 222, 239). Characterization of the HEI10 mutant allele Hei10 mei4/mei4 in mouse [not to be confused with the meiotic DSB protein MEI4 (118)] showed sterility in both males and females, along with a lack of localization of late-stage crossover class I–specific protein complex MutLγ and a severe decrease in chiasmata, with only residual class II chiasmata remaining (180, 208, 227).…”

Sordaria, a model system to uncover links between meiotic pairing and recombination

“…Recombination proteins Rad51, Mer3, Msh4 and Hei10 as well as the SC proteins Zip4 and Sme4 are loaded synchronously in both nuclei but between sister chromatids instead of between homologues (Fig. 2C; [28]). Thus, absence of karyogamy converts the entire recombination/synapsis program from an inter-homolog process to an inter-sister process.…”

“…Moreover, presence of SC components along all haploid slp1D chromosomes (Fig. 2C) revealed that each sister-chromatid can indeed serve as an LEs for SC formation [28]. …”

“…Also, clear changes in overall chromatin conformation (by DAPI) allow easy sub-staging of pachytene nuclei [25,26]. Second, contrary to most eukaryotes, the two sets of homologous chromosomes are in separated nuclei before entering meiosis [27,28]. Thus, as their first contacts occur only after karyogamy, homologous juxtaposition cannot involve contributions resulting from pre-existing pairing (review in [29,30]).…”