Mek1 kinase activity functions downstream of RED1 in the regulation of meiotic double strand break repair in budding yeast

Homologous chromosome interactions in meiosis: diversity amidst conservation

Proper chromosome segregation is crucial for preventing fertility problems, birth defects and cancer. During mitotic cell divisions, sister chromatids separate from each other to opposite poles, resulting in two daughter cells that each have a complete copy of the genome. Meiosis poses a special problem in which homologous chromosomes must first pair and then separate at the first meiotic division before sister chromatids separate at the second meiotic division. So, chromosome interactions between homologues are a unique feature of meiosis and are essential for proper chromosome segregation. Pairing and locking together of homologous chromosomes involves recombination interactions in some cases, but not in others. Although all organisms must match and lock homologous chromosomes to maintain genome integrity throughout meiosis, recent results indicate that the underlying mechanisms vary in different organisms.

A second-site suppressor strategy for chemical genetic analysis of diverse protein kinases

Chemical genetic analysis of protein kinases involves engineering kinases to be uniquely sensitive to inhibitors and ATP analogs that are not recognized by wild-type kinases. Despite the successful application of this approach to over two dozen kinases, several kinases do not tolerate the necessary modification to the ATP binding pocket, as they lose catalytic activity or cellular function upon mutation of the 'gatekeeper' residue that governs inhibitor and nucleotide substrate specificity. Here we describe the identification of second-site suppressor mutations to rescue the activity of 'intolerant' kinases. A bacterial genetic selection for second-site suppressors using an aminoglycoside kinase APH(3')-IIIa revealed several suppressor hotspots in the kinase domain. Informed by results from this selection, we focused on the beta sheet in the N-terminal subdomain and generated a structure-based sequence alignment of protein kinases in this region. From this alignment, we identified second-site suppressors for several divergent kinases including Cdc5, MEKK1, GRK2 and Pto. The ability to identify second-site suppressors to rescue the activity of intolerant kinases should facilitate chemical genetic analysis of the majority of protein kinases in the genome.

Recombination proteins in yeast

The process of homologous recombination promotes error-free repair of double-strand breaks and is essential for meiosis. Central to the process of homologous recombination are the RAD52 group genes (RAD50, RAD51, RAD52, RAD54, RDH54/TID1, RAD55, RAD57, RAD59, MRE11, and XRS2), most of which were identified by their requirement for the repair of ionizing radiation-induced DNA damage in Saccharomyces cerevisiae. The Rad52 group proteins are highly conserved among eukaryotes. Recent studies showing defects in homologous recombination and double-strand break repair in several human cancer-prone syndromes have emphasized the importance of this repair pathway in maintaining genome integrity. Herein, we review recent genetic, biochemical, and structural analyses of the genes and proteins involved in recombination.

S. pombe meiotic linear elements contain proteins related to synaptonemal complex components

The fission yeast Schizosaccharomyces pombe does not form synaptonemal complexes (SCs) in meiotic prophase nuclei. Instead, thin threads, the so-called linear elements (LEs), are observed at the corresponding stages by electron microscopy. Here, we demonstrate that S. pombe Rec10 is a protein related to the Saccharomyces cerevisiae SC protein Red1 and that it localizes to LEs. Moreover, a homologue to S. cerevisiae Hop1 does exist in S. pombe and we show by in situ immunostaining that it, and the kinase Mek1 (a homologue of which is also known to be associated with SCs), localizes to LEs. These observations indicate the evolutionary relationship of LEs with the lateral elements of SCs and suggest that these structures might exert similar functions in S. cerevisiae and S. pombe.

THE GENETICS AND MOLECULAR BIOLOGY OF THE SYNAPTONEMAL COMPLEX

The synaptonemal complex (SC) is a protein lattice that resembles railroad tracks and connects paired homologous chromosomes in most meiotic systems. The two side rails of the SC, known as lateral elements (LEs), are connected by proteins known as transverse filaments. The LEs are derived from the axial elements of the chromosomes and play important roles in chromosome condensation, pairing, transverse filament assembly, and prohibiting double-strand breaks (DSBs) from entering into recombination pathways that involve sister chromatids. The proteins that make up the transverse filaments of the SC also play a much earlier role in committing a subset of DSBs into a recombination pathway, which results in the production of reciprocal meiotic crossovers. Sites of crossover commitment can be observed as locations where the SC initiates and as immunostaining foci for a set of proteins required for the processing of DSBs to mature crossovers. In most (but not all) organisms it is the establishment of sites marking such crossover-committed DSBs that facilitates completion of synapsis (full-length extension of the SC). The function of the mature full-length SC may involve both the completion of meiotic recombination at the DNA level and the exchange of the axial elements of the two chromatids involved in the crossover. However, the demonstration that the sites of crossover formation are designated prior to SC formation, and the finding that these sites display interference, argues against a role of the mature SC in mediating the process of interference. Finally, in at least some organisms, modifications of the SC alone are sufficient to ensure meiotic chromosome segregation in the complete absence of meiotic recombination.