Identification of a Golgi-associated protein that undergoes mitosis dependent phosphorylation and relocation

Phosphorylation and redistribution of the phosphatidylinositol-transfer protein in phorbol 12-myristate 13-acetate- and bombesin-stimulated Swiss mouse 3T3 fibroblasts

By immunofluorescence microscopy it was shown that the phosphatidylinositol-transfer protein (PI-TP) becomes associated with the Golgi membranes when confluent (quiescent) Swiss mouse 3T3 fibroblast cells are stimulated with phorbol 12-myristate 13-acetate (PMA) and bombesin. Dibutyryl cyclic AMP or dexamethasone had no effect on the intracellular redistribution of PI-TP. In exponentially growing cells and in serum-starved (semi-quiescent) cells, PI-TP is already associated with Golgi structures. Stimulation of semi-quiescent cells by PMA resulted in a rapid redistribution of PI-TP. A similar yet slower response was observed after stimulation with bombesin. Stimulation of semi-quiescent 3T3 cells by PMA significantly increased the phosphorylation of PI-TP, as shown by immunoprecipitation of PI-TP from pre-labelled cells. No significant increase in phosphorylation of PI-TP was observed after stimulation of these cells by bombesin. Purified PI-TP was shown to be a substrate for protein kinase C in vitro. The possibility that the phosphorylation of PI-TP after activation of protein kinase C is involved in the observed redistribution of PI-TP is discussed.

A method for the generation of monoclonal antibodies against rare cell-surface molecules

Monoclonal antibodies provide a powerful tool for the identification and analysis of novel cell-surface molecules. We present here a method for antigen preparation and an immunization protocol that facilitates generation of mAb reactive with cell-surface molecules of low abundance and/or low antigenicity. The procedure involves isolation and extensive fractionation of cell-surface and detergent-soluble extracellular-matrix molecules prior to immunization. Cell-surface proteins on intact tissue are biotin-labeled using a reagent that does not penetrate cells. Avidin affinity chromatography is then used to purify these biotinylated molecules. Size-exclusion HPLC is used to separate these surface molecules on the basis of apparent molecular mass. Finally, immunization with antigen coupled to keyhole-limpet hemocyanin is combined with long-term booster immunizations to generate a hyperimmune response resulting in high-affinity IgG. A test application of this approach was aimed at the generation of mAb against cell-surface molecules of approximately 135 kDa in the developing chicken retinotectal system. Immunochemical analyses using antibodies produced by this approach which showed restricted patterns of tissue staining reveal that mAb were generated against all previously identified immunoglobulin superfamily molecules of this size in this system. Furthermore, we produced many additional antibodies that labeled retinotectal tissue in novel staining patterns. In the two cases analyzed in detail, these new patterns reflect the distributions of previously uncharacterized members of the immunoglobulin superfamily. The success of this initial study suggests that this method may represent a broadly applicable approach towards the preparation of extensive libraries of antibodies against cell-surface molecules expressed on cells from numerous sources.

Human cyclins A and B1 are differentially located in the cell and undergo cell cycle-dependent nuclear transport

We have used immunofluorescence staining to study the subcellular distribution of cyclin A and B1 during the somatic cell cycle. In both primary human fibroblasts and in epithelial tumor cells, we find that cyclin A is predominantly nuclear from S phase onwards. Cyclin A may associated with condensing chromosomes in prophase, but is not associated with condensed chromosomes in metaphase. By contrast, cyclin B1 accumulates in the cytoplasm of interphase cells and only enters the nucleus at the beginning of mitosis, before nuclear lamina breakdown. In mitotic cells, cyclin B1 associates with condensed chromosomes in prophase and metaphase, and with the mitotic apparatus. Cyclin A is degraded during metaphase and cyclin B1 is precipitously destroyed at the metaphase----anaphase transition. Cell fractionation and immunoprecipitation studies showed that both cyclin A and cyclin B1 are associated with PSTAIRE-containing proteins. The nuclear, but not the cytoplasmic form, of cyclin A is associated with a 33-kD PSTAIRE-containing protein. Cyclin B1 is associated with p34cdc2 in the cytoplasm. Thus we propose that the different localization of cyclin A and cyclin B1 in the cell cycle could be the means by which the two types of mitotic cyclin confer substrate specificity upon their associated PSTAIRE-containing protein kinase subunit.