Identification and characterization of synaptonemal complex proteins of the rat

Synaptonemal complexes (SCs) are structures that are formed between homologous chromosomes during meiotic prophase. They undergo a series of morphological alterations which closely correlate with the successive rearrangements of meiotic prophase chromatin, namely chromosome condensation, pairing, recombination and segregation. Despite this correlation, as yet no functions have been assigned with certainty to SCs (Chapter 1). The work described in this thesis is focused on the identification and characterization of components of SCs of the rat with the purpose of biochemical and functional analysis of SCs. For the identification of SC components we elicited monoclonal and polyclonal antibodies against isolated SCs.Chapter 2 describes the isolation of SC-specific monoclonal antibodies and the identification of four major components of the SC by means of these antibodies. Three major components of the lateral elements (LEs) were identified, with relative electrophoretic mobilities (M r s) of 30,000, 33,000 and 190,000 respectively. All 18 monoclonal antibodies that recognize the Mr 30,000 SC component also recognize the 33,000 component; apparently these components are related. One M r 125,000 component was localized at the inner edge of the LEs, specifically where chromosomes are paired (synapsed). Furthermore, we found that M r 66,000 - 55,000 antigens were localized in clusters in the vicinity of the SCs. It is possible that these antigens are SC-associated proteins, rather than real SC components.Chapter 3 describes an analysis of the tissue distribution of the Mr 30,000, 33,000, the Mr 125,000 and the M r 190,000 SC antigens. All antigens could only be detected in meiotic prophase cells by means of immunofluorescence. The M r 30,000 and 33,000 components were observed in all stages of meiotic prophase where LEs are present, i.e. from zygotene up to and including diplotene, in paired as well as unpaired segments of SCs. It could not be excluded that traces of both proteins, in a form not recognizable as SCs, were present in spermatogonia and spermatids. The tissue distribution of the M r 190,000 component, as detected by immunofluorescence, was indistinguishable from the distribution of the M r 30,000 and 33,000 antigens. The M r 125,000 component was exclusively present in meiotic prophase nuclei, in paired segments of SCs. Thus, SCs largely consist of newly synthesized proteins and the chromatin rearrangements of the meiotic prophase involve the reorganization of chromatin onto newly assembled, meiosis-specific structures, the SCs.Chapter 4 describes the isolation of cDNA clones encoding SCP2, a major component of the LEs of SCs, by means of Mabs that recognize the M r 190,000 SC component. The gene encoding SCP2 is exclusively expressed in the testis, predominantly in spermatocytes. SCP2 has a predicted molecular weight of 148 kDa. It shares some features with DNA binding proteins that are involved in chromatin organization, such as the SATB1 and RAP1 proteins: it is rich in 11turns, it has a high content of proline residues, and it has several S/T-P-X-X and S/T-S/T-X-X motifs. We speculate that SCP2 interacts directly with DNA and has a chromatin organizing function.Chapter 5 describes the isolation of cDNA clones encoding SCP1, a major component of transverse filaments of SCs, by means of a Mab that recognizes the M r 125,000 SC component. The transcription of the gene encoding SCP1 is restricted to zygotene till diplotene spermatocytes. A polyclonal antiserurn raised against the fusion protein produced by one of the cDNA clones recognizes a single protein on Western blots of isolated SCs, with identical electrophoretic mobility as the antigen recognized by the anti-M r 125,000 Mab. SCP1 has a predicted molecular weigth of 111 kDa. It shares several features with nuclear lamins and some nuclear matrix proteins. A major part of SCP1 is capable of forming an amphipathic α-helix. This region shows amino acid similarity to the coiled-coil region of myosin heavy chain. We speculate that SCP1 has evolved by specialization of a nuclear matrix protein.Chapter 6 describes the analysis of the M r 30,000 and 33,000 SC components by two- dimensional gel electrophoresis. These SC components are resolved in at least 24 spots, with pi values between 6 and>9. Some of the anti-M r 30,000 and 33,000 Mabs recognize all 24 spots, which indicates that the spots represent related proteins or variants or breakdown products of the same protein.