THE SYNAPTONEMAL COMPLEX PROTEIN SCP3 CAN FORM MULTISTRANDED, CROSS-STRIATED FIBERS IN-VIVO

The synaptonemal complex protein SCP3 is part of the lateral element o f the synaptonemal complex, a meiosis-specific protein structure essen tial for synapsis of homologous chromosomes. We have investigated the fiber-forming properties of SCP3 to elucidate its role in the synapton emal complex. By synthesis of SCP3 in cultured somatic cells, it has b een shown that SCP3 can self-assemble into thick fibers and that this process requires the COOH-terminal coiled coil domain of SCP3, as well as the NH2-terminal nonhelical domain. We have further analyzed the t hick SCP3 fibers by transmission electron microscopy and immunoelectro n microscopy, We found that the fibers display a transversal striation with a periodicity of similar to 20 nm and consist of a large number of closely associated, thin fibers, 5-10 nm in diameter. These feature s suggest that the SCP3 fibers are structurally related to intermediat e filaments. It is known that in some species the lateral elements of the synaptonemal complex show a highly ordered striated structure rese mbling that of the SCP3 fibers. We propose that SCP3 fibers constitute the core of the lateral elements of the synaptonemal complex and func tion as a molecular framework to which other proteins attach, regulati ng DNA binding to the chromatid axis, sister chromatid cohesion, synap sis, and recombination.