SYNAPTONEMAL COMPLEX PROTEINS - OCCURRENCE, EPITOPE MAPPING AND CHROMOSOME DISJUNCTION

We have used polyclonal antibodies against fusion proteins produced fr om cDNA fragments of a meiotic chromosome core protein, Cor1, and a pr otein present only in the synapsed portions of the cores, Syn1, to det ect the occurrence and the locations of these proteins in rodent meiot ic prophase chromosomes. The 234 amino acid Cor1 protein is present in early unpaired cores, in the lateral domains of the synaptonemal comp lex and in the chromosome cores when they separate at diplotene, A nov el observation showed the presence of Cor1 axial to the metaphase I ch romosomes and substantial amounts of Cor1 in association with pairs of sister centromeres. The centromere-associated Cor1 protein becomes di ssociated from the centromeres at anaphase II and it is not found in m itotic metaphase centromeres. The extended presence of Cor1 suggests t hat it may have a role in chromosome disjunction by fastening chiasmat a at metaphase I and by joining sister kinetochores, which ensures co- segregation at anaphase I. Two-colour immunofluorescence of Cor1 and S yn1 demonstrates that synapsis between homologous cores is initiated a t few sites but advances rapidly relative to the establishment of new initiation sites. If the rapid advance of synapsis deters additional i nitiation sites between pairs of homologues, it may provide a mechanis m for positive recombination interference. Immunogold epitope mapping of antibodies to four Syn1 fusion proteins places the amino terminus o f Syn1 towards the centre of the synaptonemal complex while the carbox yl terminus extends well into the lateral domain of the synaptonemal c omplex. The Syn1 fusion proteins have a non-specific DNA binding capac ity. Immunogold labelling of Cor1 antigens indicates that the lateral domain of the synaptonemal complex is about twice as wide as the appar ent width of lateral elements when stained with electron-dense metal i ons. Electron microscopy of shadow-cast surface-spread SCs confirms th e greater width of the lateral domain. The implication of these dimens ions is that the proteins that comprise the synaptic domain overlap,vi th the protein constituents of the lateral domains of the synaptonemal complex more than was apparent from earlier observations. This arrang ement suggests that direct interactions might be expected between some of the synaptonemal complex proteins.