ORGANIZATION OF SCP1 PROTEIN MOLECULES WITHIN SYNAPTONEMAL COMPLEXES OF THE RAT

SCP1, a major protein component of synaptonemal complexes (SCs), is pr obably a constituent of the transverse filaments (TFs). The protein co nsists of three domains: a short, proline-l-ich N-terminal part, a str etch of 700 amino acid residues capable of forming an amphipathic ct-h elix, and a C-terminal domain of 240 amino acid residues which is capa ble of binding to DNA. To analyze the orientation of SCP1 molecules wi thin SCs, me elicited polyclonal antibodies against three non-overlapp ing fragments of SCP1, which comprise, respectively, the N-terminus, t he C-terminus, and a fragment from the middle of the SCP1 molecule, Us ing these antibodies, we performed immunoelectron microscopy on SCs in two types of preparations, namely, surface-spread spermatocytes and u ltrathin sections of Lowicryl-embedded testicular tissue of the rat. F or each of the three antibodies used, the distribution of immunogold l abel on surface-spread spermatocytes differed significantly from the d istribution of label on sections, Masking of SCP1 epitopes within the lateral elements (LEs) and the central element (CE) of SCs in surface- spread preparations and the influence of the surface morphology of the spreads on the labeling pattern were considered as possible explanati ons for these differences, We therefore relied on the results from sec tions for the localization of epitopes. On the basis of the distributi ons of immunogold label in Lowicryl sections and the predicted seconda ry structure and dimensions of SCP1 molcules, we present the following model: the C-terminus of SCP1 molecules lies in the inner half of the LE, Cite molecules protrude from the LE through the central region in to the CE, and end up with their N-terminus between the center of the CE and the opposite LE, so that the N-termini of SCP1 molecules from o pposite LEs overlap, The model several implications for the assembly o f SCs and the possible functions of SCP1. (C) 1996 Academic Press, Inc .