Membrane and cytoplasmic proteins are transported in the same organelle complex during nematode spermatogenesis.

During the development of pseudopodial spermatozoa of the nematode, Caenorhabditis elegans, protein synthesis stops before differentiation is completed. Colloidal gold conjugates of monoclonal antibody SP56, which binds to the surface of spermatozoa, and TR20, which recognizes the major sperm cytoplasmic protein (MSP), were used to label thin sections of testes embedded in Lowicryl K4M in order to follow polypeptides from their synthesis early in spermatogenesis to their segregation to specific compartments of the mature cell. Both antigens are synthesized in primary spermatocytes and are assembled into a unique double organelle, the fibrous body-membranous organelle (FB-MO) complex. However, the antigens are localized in different regions of this FB-MO complex. As described in detail, the assembly of proteins into the FB-MO complex allows both membrane and cytoplasmic components to be concentrated in the spermatids after meiosis. Then, the stepwise disassembly of this transient structure ensures delivery of each component to its final destination in the mature spermatozoan: MSP filaments in the fibrous body depolymerize, releasing MSP into the cytoplasm and the membranous organelles fuse with the plasma membrane, delivering SP56 antigen to the surface.