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Binding between DIP and Dpr neuronal recognition proteins has been proposed to regulate synaptic connections between lamina and medulla neurons in the Drosophila visual system. Each lamina neuron was previously shown to express many Dprs. Here, we demonstrate, by contrast, that their synaptic partners typically express one or two DIPs, with binding specificities matched to the lamina neuron-expressed Dprs. A deeper understanding of the molecular logic of DIP/Dpr interaction requires quantitative studies on the properties of these proteins. We thus generated a quantitative affinity-based DIP/Dpr interactome for all DIP/Dpr protein family members. This revealed a broad range of affinities and identified homophilic binding for some DIPs and some Dprs. These data, along with full-length ectodomain DIP/Dpr and DIP/DIP crystal structures, led to the identification of molecular determinants of DIP/Dpr specificity. This structural knowledge, along with a comprehensive set of quantitative binding affinities, provides new tools for functional studies invivo.

Replicated sister chromatids are held in close association from the time of their synthesis until their separation during the next mitosis. This association is mediated by the ring-shaped cohesin complex that appears to embrace the sister chromatids. Upon proteolytic cleavage of the α-kleisin cohesin subunit at the metaphase-to-anaphase transition by separase, sister chromatids are separated and segregated onto the daughter nuclei. The more complex segregation of chromosomes during meiosis is thought to depend on the replacement of the mitotic α-kleisin cohesin subunit Rad21/Scc1/Mcd1 by the meiotic paralog Rec8. In Drosophila, however, no clear Rec8 homolog has been identified so far. Therefore, we have analyzed the role of the mitotic Drosophila α-kleisin Rad21 during female meiosis. Inactivation of an engineered Rad21 variant by premature, ectopic cleavage during oogenesis results not only in loss of cohesin from meiotic chromatin, but also in precocious disassembly of the synaptonemal complex (SC). We demonstrate that the lateral SC component C(2)M can interact directly with Rad21, potentially explaining why Rad21 is required for SC maintenance. Intriguingly, the experimentally induced premature Rad21 elimination, as well as the expression of a Rad21 variant with destroyed separase consensus cleavage sites, do not interfere with chromosome segregation during meiosis, while successful mitotic divisions are completely prevented. Thus, chromatid cohesion during female meiosis does not depend on Rad21-containing cohesin., Author Summary Meiosis is a specialized form of cell division that ensures production of germ cells with the right number of chromosomes, so that at fertilization the embryo receives complete sets of paternal and maternal chromosomes. The accurate distribution of chromosomes during cell divisions is dependent on a ring-shaped protein complex called cohesin. Cohesin is thought to embrace the chromosomes from the time of their duplication during S-phase until their segregation in the ensuing division. This segregation is facilitated by the controlled proteolytic cleavage of one of the cohesin ring components. Most eukaryotes express specialized variants of this protein: for mitosis the variant Rad21/Scc1/Mcd1 and for meiosis the related protein Rec8. Because Drosophila lacks a clear Rec8 homolog, we have analyzed in the present study whether the mitotic variant Rad21 may also function during meiosis. We have destroyed Rad21-based cohesin by premature cleavage of an engineered Rad21 variant during oogenesis. While we find no indication for effects on the accuracy of meiotic chromosome segregation, Rad21 cleavage results in a premature disassembly of the synaptonemal complex (SC), a structure required for meiotic recombination in Drosophila oocytes. Our interaction studies provide intriguing hints how Rad21 might contribute to SC maintenance.