Juxtaposition of C(2)M and the transverse filament protein C(3)G within the central region of Drosophila synaptonemal complex

Abstract

The synaptonemal complex (SC) is intimately involved in the process of meiotic recombination in most organisms, but its exact role remains enigmatic. One reason for this uncertainty is that the overall structure of the SC is evolutionarily conserved, but many SC proteins are not. Two putative SC proteins have been identified in Drosophila: C(3)G and C(2)M. Mutations in either gene cause defects in SC structure and meiotic recombination. Although neither gene is well conserved at the amino acid level, the predicted secondary structure of C(3)G is similar to that of transversefilament proteins, and C(2)M is a distantly related member of the ␣-kleisin family that includes Rec8, a meiosis-specific cohesin protein. Here, we use immunogold labeling of SCs in Drosophila ovaries to localize C(3)G and C(2)M at the EM level. We show that both C(3)G and C(2)M are components of the SC, that the orientation of C(3)G within the SC is similar to other transverse-filament proteins, and that the N terminus of C(2)M is located in the central region adjacent to the lateral elements (LEs). Based on our data and the known phenotypes of C(2)M and C(3)G mutants, we propose a model of SC structure in which C(2)M links C(3)G to the LEs. meiosis ͉ recombination ͉ chromosome ͉ immunogold ͉ electron microscopy I n general terms, the structure of the synaptonemal complex (SC) is conserved among diverse organisms with two lateral elements (LEs) that run along the length of each pair of homologous chromosomes, a central element (CE) that is located midway between the two LEs, and transverse filaments (TF) that connect the LEs to the CE (reviewed in ref. 1). However, distinct differences exist among organisms, particularly in the degree of organization of the CE (2). The conditions required for SC assembly also differ, with DNA double-strand breaks being required for SC formation in some species (e.g., budding yeast, mammals, and plants) but not in others (Drosophila and Caenorhabditis elegans) (3-5). These differences may be useful in defining nonconserved features of SC as well as in highlighting conserved functions.The morphological structures of CEs from a mammal (rat) and two insects (Drosophila and a beetle, Blaps cribrosa) were analyzed at high resolution by using EM tomography (2). In these organisms, the CE structure is essentially the same, but the degree of organization varies considerably. The CE in insects is highly organized, with two (and sometimes more) distinct longitudinal components. These dense longitudinal components appear to be composed of vertical ''pillars'' that link multiple layers of CE together. In comparison, the CE of mammals is less well organized; multiple layers of CE are not obvious, and the longitudinal components are so discontinuous that they typically appear as a single, rather broad, dark structure midway between LEs (2, 6). Some investigators have suggested that the longitudinal components are formed, at least partially, by the Nterminal domains of TFs (7, 8). Whether this difference among species in th...

Evolutionary history of the mammalian synaptonemal complex

“…C(2)M is localized at the inner side of the AEs/LEs of the SC close to the C-termini of C(3)G during prophase I (Anderson et al 2005). The proteins show the same localization pattern in immunofluorescence analyses and C(2)M is proposed to function as linker between the TFs and the LEs (Manheim and Mckim 2003;Anderson et al 2005).…”

“…The proteins show the same localization pattern in immunofluorescence analyses and C(2)M is proposed to function as linker between the TFs and the LEs (Manheim and Mckim 2003;Anderson et al 2005). Besides C(2)M, the LEs are composed by several proteins including cohesins, the ORD protein and Nipped-B (Bickel et al 1996; The central region is formed by SYCP1 dimers which make the transverse filaments and the proteins of the central element -SYCE1, SYCE2, SYCE3 and Tex12.…”

“…However, the different protein components of the respective domains (LEs, TFs and CE) exhibit great primary amino acid sequence diversities (e.g. Page and Hawley 2004;Anderson et al 2005;Bogdanov et al 2007;Bolcun-Filas and Schimenti 2012). This lack of sequence identity 7 is surprising considering a single origin of meiosis in general and, in particular, the structural conservation of the SC.…”

“…C(3)G is 744 aa long and has a predicted central rod domain from aa 158 to aa 646 which is flanked by globular termini (Page and Hawley 2001). Within the SC, the protein spans the distance between the LEs and the CE and exhibits the same orientation with its N-terminus in the center of the SC and the C-terminus in the LEs (Anderson et al 2005). The N-terminus was found to be essential for polymerization of C(3)G molecules to dimers and tetramers in the CR and also for polycomplex formation in oocytes.…”

Functional links between Drosophila Nipped-B and cohesin in somatic and meiotic cells

“…6). It also colocalizes with the Ord protein, which is required for meiotic cohesion in both sexes and SC maintenance in oocytes (Bickel et al 1997;Khetani and Bickel 2007;Webber et al 2004), and with C(3)G, a transverse filament protein that resides within the central element of the SC (Anderson et al 2005). These data argue that Nipped-B may play a direct role in regulating meiotic cohesion as well as SC structure and function.…”