Synaptonemal complexes (SCs) are structures that are formed between homologous chromosomes during meiotic prophase. They are probably involved in chromosome pairing and recombination. Using a monoclonal anti‐SC antibody we isolated cDNAs encoding a major component of SCs which is localized specifically in synapsed segments of meiotic prophase chromosomes. The protein predicted from the nucleotide sequence of a full‐length cDNA, named SCP1, consists of 946 amino acid residues and has a molecular weight of 111 kDa. It shares several features with nuclear lamins and some recently identified nuclear matrix proteins. The major part of SCP1 consists of long stretches capable of forming amphipathic alpha‐helices. This region shows amino acid sequence similarity to the coiled‐coil region of myosin heavy chain. A leucine zipper is included in this region. The carboxy‐terminus has two small basic domains and several S/T‐P‐X‐X motifs, which are characteristic of DNA‐binding proteins. One of these motifs is a potential target site for p34cdc2 protein kinase. The amino‐terminus is acidic and relatively proline‐rich, but does not contain the S/T‐P‐X‐X motif. The transcription of the gene encoding SCP1 is restricted to zygotene‐diplotene spermatocytes. A polyclonal antiserum raised against the fusion protein of one of the cDNA clones recognizes a single protein on Western blots of isolated SCs, with an electrophoretic mobility identical to that of the antigen recognized by the original monoclonal antibody (mAb), IX5B2. From a detailed comparison of the immunogold labelling of rat SCs by mAb IX5B2 and the polyclonal anti‐fusion protein antiserum respectively, we tentatively infer that the carboxy‐terminus of SCP1 is orientated towards the lateral elements and that the other domains of the protein extend towards the central region between the lateral elements. We conclude that SCP1 is the major component of the transverse filaments of SCs, and speculate that it has evolved by specialization of a nuclear matrix protein.

The lateral elements of synaptonemal complexes (SCs) of the rat contain major components with relative electrophoretic mobilities (Mrs) (46,53). For the proper orientation of bivalents, sister chromatid cohesiveness, which persists until anaphase I, is also essential (24,30,41).At the DNA level, the major events of meiotic prophase result in a high rate of homologous recombination and gene conversion. Meiotic and mitotic recombination differ in several respects. (i) The rate of meiotic reciprocal exchange (and of gene conversion) is several orders of magnitude higher than that of mitotic exchange. (ii) With few exceptions (62), there is positive interference between meiotic reciprocal exchanges (reviewed in references 50 and 67), whereas no evidence has been obtained for interference between mitotic reciprocal exchanges (32). (iii) In several organisms, hot spots for meiotic reciprocal recombination (and gene conversion) have been identified (6,18,22,57), but most of these are not hot spots for mitotic recombination (reviewed in reference 50). (iv) Meiotic reciprocal recombination occurs preferentially between non-sister chromatids of homologous chromosomes (19,64), whereas the sister chromatid is preferred for mitotic exchange (31). Thus, there is a meiosis-specific chromatin organization by which some sequences (hot spots) are preferentially exposed for homology searching and recombination, positive interference between reciprocal exchanges is effected, reciprocal exchanges between sister chromatids are avoided (or exchanges between non-sister chromatids are preferentially enhanced), sister chromatid cohesiveness is brought about, and reciprocal exchanges between non-sister chromatids result in the formation of functional chiasmata.It seems likely that SCs, particularly the LEs, play a role in one or more of these aspects of meiosis-specific chromatin 1137

During meiosis, homologous chromosomes (homologs) undergo recombinational interactions, which can yield crossovers (COs) or noncrossovers. COs exhibit interference; they are more evenly spaced along the chromosomes than would be expected if they were placed randomly. The protein complexes involved in recombination can be visualized as immunofluorescent foci. We have analyzed the distribution of such foci along meiotic prophase chromosomes of the mouse to find out when interference is imposed and whether interference manifests itself at a constant level during meiosis. We observed strong interference among MLH1 foci, which mark CO positions in pachytene. Additionally, we detected substantial interference well before this point, in late zygotene, among MSH4 foci, and similarly, among replication protein A (RPA) foci. MSH4 foci and RPA foci both mark interhomolog recombinational interactions, most of which do not yield COs in the mouse. Furthermore, this zygotene interference did not depend on SYCP1, which is a transverse filament protein of mouse synaptonemal complexes. Interference is thus not specific to COs but may occur in other situations in which the spatial distribution of events has to be controlled. Differences between the distributions of MSH4͞RPA foci and MLH1 foci along synaptonemal complexes might suggest that CO interference occurs in two successive steps.crossing-over ͉ immunofluorescence ͉ meiosis M eiosis consists of two divisions, meiosis I and II, by which a diploid cell produces four haploid daughters. Reduction in ploidy occurs at meiosis I, when homologous chromosomes (homologs) disjoin. This event is prepared during meiotic prophase, when homologs recognize each other and form stable pairs (bivalents) that can line up in the metaphase I spindle. In most eukaryotes, including mouse and yeast, both the recognition of homologs and the formation of stable bivalents depend on recombinational interactions between homologs (reviewed in ref. 1). For this process, the meiotic prophase cell actively induces DNA double-strand breaks (DSBs) and repairs them by homologous recombination, using preferably a nonsister chromatid of the homolog as template (2). In species such as yeast and mouse, most interhomolog recombinational interactions are not resolved as reciprocal exchanges [crossovers (COs)] and probably serve homolog recognition and alignment (3, 4). A small proportion, however, yields COs, which become cytologically visible as chiasmata and are essential for the stable connection of homologs. COs are not randomly distributed among and along bivalents; every bivalent forms at least one CO (obligate CO), and, if multiple COs occur, they are more evenly spaced along the bivalent than would be expected if they were randomly placed. This phenomenon was originally detected genetically by the finding that the frequency of double recombinants involving a pair of adjacent or nearby intervals was lower than the frequency expected from recombinant frequencies for each of those intervals (reviewed in refs. 5 ...

In the axial elements of synaptonemal complexes (SCs) of the rat, major protein components have been identified, with relative electrophoretic mobilities (M rs) of 30 000-33 000 and 190 000. Using monoclonal anti-SC antibodies, we isolated cDNA fragments which encode the 190 000 M r component of rat SCs. The translation product predicted from the nucleotide sequence of the cDNA, called SCP2 (for synaptonemal complex protein 2), is a basic protein (pI = 8.0) with a molecular mass of 173 kDa. At the C-terminus, a stretch of approximately 50 amino acid residues is predicted to be capable of forming coiled-coil structures. SCP2 contains two clusters of S/T-P motifs, which are common in DNA-binding proteins. These clusters flank the central, most basic part of the protein (pI = 9.5). Three of the S/T-P motifs are potential target sites for p34(cdc2) protein kinase. In addition, SCP2 has eight potential cAMP/cGMP-dependent protein kinase target sites. The gene encoding SCP2 is transcribed specifically in the testis, in meiotic prophase cells. At the amino acid sequence and secondary structural level, SCP2 shows some similarity to the Red1 protein, which is involved in meiotic recombination and the assembly of axial elements of SCs in yeast. We speculate that SCP2 is a DNA-binding protein involved in the structural organization of meiotic prophase chromosomes.

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