The pairing of homologous chromosomes during meiotic prophase
culminates in formation of the synaptonemal complex (SC). Each SC
consists of two parallel lateral elements, corresponding to the
proteinaceous scaffolds of the individual chromosomes within the
complex. Between the lateral elements is the central region of the SC,
which contains transverse elements that lie perpendicular to the lateral
elements. A component of transverse elements of the yeast SC is the Zip1
protein; Zip1 is comprised of a long stretch of heptad repeats capable
of forming an alpha-helical coiled coil, flanked by N- and C-terminal
globular domains. Epitope mapping using antibodies specific for distinct
structural domains of Zip1 has revealed that the N-terminal domain of
Zip1 lies in the middle of the central region while the C-terminal
domain is embedded in the lateral elements. These results suggest that
two Zip1 dimers, lying head to head, span the entire width of the SC.
The N-terminal domains of Zip1 dimers from opposing lateral elements may
meet, and perhaps overlap, in the middle of the SC central region.
Purified Zip1 proteins have been shown to assemble into dimers and
tetramers in vitro under physiological conditions. Electron microscopic
analysis of wild-type and truncated versions of Zip1 dimers demonstrated
that the two Zip monomers within a dimer are parallel and in register.
The configuration of dimers within a tetramer is being investigated.
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