The proliferation-associated nuclear protein Ki-67 in the bovine system: partial characterisation and its application for determination of the proliferation of Theileria-infected bovine cells

Theileria annulata-infected bovine cells as well as mitogen-stimulated bovine peripheral blood mononuclear cells (PBMC) express a proliferation-associated nuclear protein equivalent to the human Ki-67 protein. In analogy to the human system, the expression of the bovine Ki-67 protein is restricted to proliferating cells only, since (a) Ki-67 expression paralleled [3H]-thymidine incorporation in concanavalin A (Con A)-stimulated bovine PBMC, (b) Ki-67 was not detectable in quiescent bovine cells, and (c) Ki-67 expression in Theileria-infected cells is related to the presence of the parasites within the cytoplasm of the host cells; upon treatment with the theilericidal drug buparvaquone the parasites are destroyed and the cells cease to proliferate and to express the Ki-67 protein. Western-blot analysis of lysates of proliferating bovine cells revealed that the prototype monoclonal antibody Ki-67 and the new equivalent antibody MIB-1 detected one prominent protein band with an apparent molecular weight of 430 kDa. Two cDNA clones (pUC18.B1.Ki-67 and pUC18.B2.Ki-67) were isolated from a lambdagt11 cDNA library of T. annulata-infected bovine cells by immunoscreening with the monoclonal antibody MIB-1. Comparison of these cDNA sequences with those of the human Ki-67 protein revealed 60-70% identity. Within the "Ki-67 motif", identity proved to be 80% at the amino acid level. The remarkable identity between bovine and human Ki-67 proteins suggests that MIB-1 can be used as a marker for cell proliferation in animal research. In this context we could identify proliferating cells in lymph nodes of Theileria-infected animals and, furthermore, we could distinguish between infected and uninfected proliferating cells using MIB-1 and an antiserum against a recombinant parasite protein designated SA288.

Malaria circumsporozoite protein inhibits protein synthesis in mammalian cells

Native Plasmodium circumsporozoite (CS) protein, translocated by sporozoites into the cytosol of host cells, as well as recombinant CS constructs introduced into the cytoplasm by liposome fusion or transient transfection, all lead to inhibition of protein synthesis in mammalian cells. The following findings suggest that this inhibition of translation is caused by a binding of the CS protein to ribosomes. (i) The distribution of native CS protein translocated by sporozoites into the cytoplasm as well as microinjected recombinant CS protein suggests association with ribosomes. (ii) Recombinant CS protein binds to RNase-sensitive sites on rough microsomes. (iii) Synthetic peptides representing the conserved regions I and II-plus of the P.falciparum CS protein displace recombinant CS protein from rough microsomes with dissociation constants in the nanomolar range. (iv) Synthetic peptides representing region I from the P.falciparum CS protein and region II-plus from the P.falciparum, P.berghei or P.vivax CS protein inhibit in vitro translation. We propose that Plasmodium manipulates hepatocyte protein synthesis to meet the requirements of a rapidly developing schizont. Since macrophages appear to be particularly sensitive to the presence of CS protein in the cytosol, inhibition of translation may represent a novel immune evasion mechanism of Plasmodium.

Release of malaria circumsporozoite protein into the host cell cytoplasm and interaction with ribosomes

To date, the circumsporozoite (CS) protein has been implicated in guiding malaria sporozoites to the liver [Cerami et al., Cell 70, 1992, 1021-1033]. Here we show that shortly after invasion, P. berghei and P. yoelii sporozoites lie free in the invaded cell and release considerable amounts of CS protein into the cytoplasm. The intracytoplasmic deposition of CS protein begins during the attachment of the sporozoite to the host cell surface and reaches its peak during the first 4-6 h after invasion. Initially, the CS protein spreads over the entire cytoplasm of the infected cell where it interacts with cytosolic as well as endoplasmic reticulum-associated ribosomes. During the subsequent development of the parasites to exoerythrocytic forms, the CS protein binding becomes gradually restricted to ribosomes lining the outer membrane of the nuclear envelope of the host cell. The distribution pattern of the parasite-released CS protein in the host cell cytoplasm is independent of the permissiveness of the host cell for the development of the parasites to exoerythrocytic forms. It requires neither the host cell metabolism nor does it involve the endocytotic machinery. Recombinant P. falciparum CS protein interacts with RNAse-sensitive sites on endoplasmic reticulum-associated ribosomes as shown by microinjection and immunoelectron microscopy. The generalized interaction of the CS protein with host cell ribosomes suggests that the CS protein has an intracellular function during the hepatic phase in the life cycle of Plasmodium and may also explain the generation of a CD8+ T cell response in the course of rodent malaria infections.