Freeze-fracture study of the Gregarine trophozoite: I the top of the epicyte folds

Abstract

The fine organization of the "membrane complex" present at the tip of the epicyte folds in the sporozoan Greguritia trophozoite has been studied. Electron microscopy revealed bundles of intermcdiate filaments (8-10 nm) in the interspace between plasma and middle membrane. Doublets of filaments are present in the cytoplasm adhering to the i n n s membrane. The narrow space between the inner and middle membrane exhibits electron dense dots arranged in pairs. Freeze-fracture replicas reveal a different organization in each of the three membranes. The plasma membrane shows the Pface rich in particles and E-face with a few clusters of intramembrane particles (IhlPs). Parallel ridges are present on the E-face and parallel grooves on the P-face of the intermediate membrane. The P-face of the inner membrane shows double longitudinal arrays of IhlPs and the E-face has large grooves alternating with rows of ridges. The rows of particles on the P-face of the inner membrane are in register with the clusters of filaments seen in the cytoplasm and adhering to the inner membrane.Our observations suggest connections among the three membranes at the tip of the epicyte folds; moreover, an interaction occurs between the intermediate filaments and intramembrane complexes. The arrangement of the membranes is discussed in reference to the gliding of gregames. ACKNOWLEDGEMENTS

The enigma of eugregarine epicytic folds: where gliding motility originates?

“…It could be expected that enigmatic 12-nm filaments, running under the IMC and exhibiting the properties of intermediate filaments [ 34 , 39 ], could support the actomyosin motor in a similar way. Longitudinal arrays of IMP found in the area of the 12-nm filaments and the rippled dense structures [ 11 , 22 , 30 , 40 ] are comparable to the lines of higher particle density overlaying the subpellicular microtubules in Eimeria or Plasmodium sporozoites [ 41 , 42 ]. Our data show, however, that the number of 12-nm filaments does not influence the speed of gregarine gliding (up to 7 filaments in G. cuneata vs. 10 filaments in G. polymorpha and maximally 4 filaments in G. steini ), but rather seem to control the direction of movement.…”

“…King and Sleep [ 15 ] estimated that the number of myosin heads at the site of interaction in Gregarina gamonts to be in excess of 200 and showed that the gliding rate in a giant eugregarine Porospora gigantea is four times higher (up to 60 μm/s) than the speed of myosin movement along the actin filaments of a muscle sarcomere. In spite of a few freeze-etching studies that focus on the supramolecular cell organisation of some Gregarina species [ 11 , 22 , 30 ], the precise location of the actomyosin complex is as yet unknown. However, the TRAP or TgMIC2 molecules that are in contact with the substrate suggest that the concept of a glideosome might help shed light on the role of the mucus [ 12 ] in the gliding mechanism of gregarines [ 31 - 33 ].…”

Freeze-fracture study of the Gregarine trophozoite: II. Evidence of “rosette” organization on cytomembranes in relation with micropore structure

“…The organization of the intramembrane particles (IMPs) at the tip of the folds has already been described (SchrCvel et al, 1983;Dallai & Vegni Talluri, 1983). Replicas of the surface of the epicytic folds show that the fracture plane cleaves either the top or the bottom of the folds ( trophozoite.…”

Motility in blastogregarines (Apicomplexa): Native and drug-induced organisation of Siedleckia nematoides cytoskeletal elements

“…Thus, these pores, observed only in some SEM preparations, were most likely visualised due to a specific fixative osmolarity. In eugregarines, the micropores are often located at the base of the grooves between the epicytic folds, while the smaller pores are randomly distributed on the base or on the lateral side of the folds [ 3 , 17 , 18 , 78 ]. The diameter sizes of pores in S .…”