Rab9 functions in transport between late endosomes and the trans Golgi network

Rab proteins represent a large family of ras-like GTPases that regulate distinct vesicular transport events at the level of membrane targeting and/or fusion. We report here the primary sequence, subcellular localization and functional activity of a new member of the rab protein family, rab9. The majority of rab9 appears to be located on the surface of late endosomes. Rab9, purified from Escherichia coli strains expressing this protein, could be prenylated in vitro in the presence of cytosolic proteins and geranylgeranyl diphosphate. In vitro-prenylated rab9 protein, but not C-terminally truncated rab9, stimulated the transport of mannose 6-phosphate receptors from late endosomes to the trans Golgi network in a cell-free system that reconstitutes this transport step. Rab7, a related rab protein that is also localized to late endosomes, was inactive in the in vitro transport assay, despite its efficient prenylation and capacity to bind and hydrolyze GTP. These results strongly suggest that rab9 functions in the transport of mannose 6-phosphate receptors between late endosomes and the trans Golgi network. Moreover, our results confirm the observation that a given organelle may bear multiple rab proteins with different biological functions.

Lysosome biogenesis requires Rab9 function and receptor recycling from endosomes to the trans-Golgi network

Newly synthesized lysosomal enzymes bind to mannose 6-phosphate receptors (MPRs) in the TGN, and are carried to prelysosomes, where they are released. MPRs then return to the TGN for another round of transport. Rab9 is a ras-like GTPase which facilitates MPR recycling to the TGN in vitro. We show here that a dominant negative form of rab9, rab9 S21N, strongly inhibited MPR recycling in living cells. The block was specific in that the rates of biosynthetic protein transport, fluid phase endocytosis and receptor-mediated endocytosis were unchanged. Expression of rab9 S21N was accompanied by a decrease in the efficiency of lysosomal enzyme sorting. Cells compensated for the presence of the mutant protein by inducing the synthesis of both soluble and membrane-associated lysosomal enzymes, and by internalizing lysosomal enzymes that were secreted by default. These data show that MPRs are limiting in the secretory pathway of cells expressing rab9 S21N and document the importance of MPR recycling and the rab9 GTPase for efficient lysosomal enzyme delivery.

Membrane targeting of the small GTPase Rab9 is accompanied by nucleotide exchange

The Rab GTPases are key regulators of vesicular transport. A fraction of Rab proteins is present in the cytosol, bound with GDP, complexed to a protein termed GDI. Rab9 is localized primarily to late endosomes, where it aids the transport of mannose 6-phosphate receptors to the trans-Golgi network. It has been proposed that Rab proteins are delivered to specific membranes by GDI, and that this process is accompanied by the exchange of bound GDP for GTP. In addition, Rab localization requires carboxy-terminal prenylation and specific structural determinants. Here we describe the reconstitution of the selective targeting of prenylated Rab9 protein onto late endosome membranes and show that this process is accompanied by endosome-triggered nucleotide exchange.

Ca2+-triggered peptide secretion in single cells imaged with green fluorescent protein and evanescent-wave microscopy

Green fluorescent protein fused to human chromogranin B or neuropeptide Y was expressed in PC12 cells and caused bright, punctate fluorescence. The fluorescent points colocalized with the endogenous secretory granule marker dopamine beta-hydroxylase. Stimulation of live PC12 cells with elevated [K+], or of permeabilized PC12 cells with Ca2+, led to Ca2+-dependent loss of fluorescence from neurites. Ca2+ stimulated secretion of both fusion proteins equally well. In living cells, single fluorescent granules were imaged by evanescent-wave fluorescence microscopy. Granules were seen to migrate; to stop, as if trapped by plasmalemmal docking sites; and then to disappear abruptly, as if through exocytosis. Evidently, GFP fused to secreted peptides is a fluorescent marker for dense-core secretory granules and may be used for time-resolved microscopy of single granules.

Role of actin cortex in the subplasmalemmal transport of secretory granules in PC-12 cells

In neuroendocrine PC-12 cells, evanescent-field fluorescence microscopy was used to track motions of green fluorescent protein (GFP)-labeled actin or GFP-labeled secretory granules in a thin layer of cytoplasm where cells adhered to glass. The layer contained abundant filamentous actin (F-actin) locally condensed into stress fibers. More than 90% of the granules imaged lay within the F-actin layer. One-third of the granules did not move detectably, while two-thirds moved randomly; the average diffusion coefficient was 23 x 10(-4) microm(2)/s. A small minority (<3%) moved rapidly and in a directed fashion over distances more than a micron. Staining of F-actin suggests that such movement occurred along actin bundles. The seemingly random movement of most other granules was not due to diffusion since it was diminished by the myosin inhibitor butanedione monoxime, and blocked by chelating intracellular Mg(2+) and replacing ATP with AMP-PNP. Mobility was blocked also when F-actin was stabilized with phalloidin, and was diminished when the actin cortex was degraded with latrunculin B. We conclude that the movement of granules requires metabolic energy, and that it is mediated as well as limited by the actin cortex. Opposing actions of the actin cortex on mobility may explain why its degradation has variable effects on secretion.

Rab GDI: a solubilizing and recycling factor for rab9 protein

Rab proteins are thought to function in the processes by which transport vesicles identify and/or fuse with their respective target membranes. The bulk of these proteins are membrane associated, but a measurable fraction can be found in the cytosol. The cytosolic forms of rab3A, rab11, and Sec4 occur as equimolar complexes with a class of proteins termed "GDIs," or "GDP dissociation inhibitors." We show here that the cytosolic form of rab9, a protein required for transport between late endosomes and the trans Golgi network, also occurs as a complex with a GDI-like protein, with an apparent mass of approximately 80 kD. Complex formation could be reconstituted in vitro using recombinant rab9 protein, cytosol, ATP, and geranylgeranyl diphosphate, and was shown to require an intact rab9 carboxy terminus, as well as rab9 geranylgeranylation. Monoprenylation was sufficient for complex formation because a mutant rab9 protein bearing the carboxy terminal sequence, CLLL, was prenylated in vitro by geranylgeranyl transferase I and was efficiently incorporated into 80-kD complexes. Purified, prenylated rab9 could also assemble into 80-kD complexes by addition of purified, rab3A GDI. Finally, rab3A-GDI had the capacity to solubilize rab9GDP, but not rab9GTP, from cytoplasmic membranes. These findings support the proposal that GDI proteins serve to recycle rab proteins from their target membranes after completion of a rab protein-mediated, catalytic cycle. Thus GDI proteins have the potential to regulate the availability of specific intracellular transport factors.

Morbidity and mortality after D2 gastrectomy for gastric cancer: results of the Italian Gastric Cancer Study Group prospective multicenter surgical study

PURPOSE

To investigate whether pancreas preservation together with a strict quality-control system could ameliorate the outcome of D2 resections for gastric cancer in Western patients.

PATIENTS AND METHODS

Italian patients with potentially curable proven adenocarcinoma of the stomach were registered from nine general and/or university hospitals in the area of Turin, Northern Italy. The study was performed according to the guidelines of the Japanese Research Society for Gastric Cancer (JRSGC). A strict quality-control system was guaranteed by a supervising surgeon of the reference center, who had stayed at the National Cancer Center Hospital, Tokyo, to learn the standard D2 gastrectomy. The standard procedure entailed removal of the level 1 and 2 lymph nodes. During total gastrectomy, the pancreas was preserved according to the Maruyama technique.

RESULTS

Between May 1994 and December 1996, 191 eligible patients were entered onto the study. The mean number of lymph nodes removed was 39. The overall morbidity rate was 20.9%. Surgical complications were observed in 16.7% of patients. Reoperation was necessary in six patients and was always successful. The overall hospital mortality rate was 3.1%; it was higher after total gastrectomy (7.46%) than after distal gastrectomy (0.8%). The average length of hospital stay was 17 days.

CONCLUSION

Given that postoperative morbidity and mortality rates are favorably comparable with those reported after the Western standard gastrectomy, the more extensive Japanese procedure with pancreas preservation can be regarded as a safe radical treatment of gastric cancer for selected Western patients treated in experienced centers.

A dibasic motif in the tail of a class XIV apicomplexan myosin is an essential determinant of plasma membrane localization

Obligate intracellular parasites of the phylum Apicomplexa exhibit gliding motility, a unique form of substrate-dependent locomotion essential for host cell invasion and shown to involve the parasite actin cytoskeleton and myosin motor(s). Toxoplasma gondii has been shown to express three class XIV myosins, TgM-A, -B, and -C. We identified an additional such myosin, TgM-D, and completed the sequences of a related Plasmodium falciparum myosin, PfM-A. Despite divergent structural features, TgM-A purified from parasites bound actin in an ATP-dependent manner. Isoform-specific antibodies revealed that TgM-A and recombinant mycTgM-A were localized right beneath the plasma membrane, and subcellular fractionation indicated a tight membrane association. Recombinant TgM-D also had a peripheral although not as sharply defined localization. Truncation of their respective tail domains abolished peripheral localization and tight membrane association. Conversely, fusion of the tails to green fluorescent protein (GFP) was sufficient to confer plasma membrane localization and sedimentability. The peripheral localization of TgM-A and of the GFP-tail fusion did not depend on an intact F-actin cytoskeleton, and the GFP chimera did not localize to the plasma membrane of HeLa cells. Finally, we showed that the specific localization determinants were in the very C terminus of the TgM-A tail, and site-directed mutagenesis revealed two essential arginine residues. We discuss the evidence for a proteinaceous plasma membrane receptor and the implications for the invasion process.

Dominant negative effect of cytoplasmic actin isoproteins on cardiomyocyte cytoarchitecture and function

The intracompartmental sorting and functional consequences of ectopic expression of the six vertebrate actin isoforms was investigated in different types of cultured cells. In transfected fibroblasts all isoactin species associated with the endogenous microfilament cytoskeleton, even though cytoplasmic actins also showed partial localization to peripheral submembranous sites. Functional and structural studies were performed in neonatal and adult rat cardiomyocytes. All the muscle isoactin constructs sorted preferentially to sarcomeric sites and, to a lesser extent, also to stress-fiber-like structures. The expression of muscle actins did not interfere with cell contractility, and did not disturb the localization of endogenous sarcomeric proteins. In sharp contrast, ectopic expression of the two cytoplasmic actin isoforms resulted in rapid cessation of cellular contractions and induced severe morphological alterations characterized by an exceptional outgrowth of filopodia and cell flattening. Quantitative analysis in neonatal cardiomyocytes indicated that the levels of accumulation of the different isoactins are very similar and cannot be responsible for the observed isoproteins-specific effects. Structural analysis revealed a remodeling of the cytoarchitecture including a specific alteration of sarcomeric organization; proteins constituting the sarcomeric thin filaments relocated to nonmyofibrillar sites while thick filaments and titin remained unaffected. Experiments with chimeric proteins strongly suggest that isoform specific residues in the carboxy-terminal portion of the cytoplasmic actins are responsible for the dominant negative effects on function and morphology.

Ethane-freezing/methanol-fixation of cell monolayers: a procedure for improved preservation of structure and antigenicity for light and electron microscopies

In order to dissect at the ultrastructural level the morphology of highly dynamic processes such as cell motility, membrane trafficking events, and organelle movements, it is necessary to fix/stop time-dependent events in the millisecond range. Ideally, immunoelectron microscopical labeling experiments require the availability of high-affinity antibodies and accessibility to all compartments of the cell. The biggest challenge is to define an optimum between significant preservation of the antigenicity in the fixed material without compromising the intactness of fine structures. Here, we present a procedure which offers an opportunity to unify preparation of cell monolayers for immunocytochemistry in fluorescence and electron microscopy. This novel strategy combines a rapid ethane-freezing technique with a low temperature methanol-fixation treatment (EFMF) and completely avoids chemical fixatives. It preserves the position and delicate shape of cells and organelles and leads to improved accessibility of the intracellular antigens and to high antigenicity preservation. We illustrate the establishment of this procedure using Dictyostelium discoideum, a powerful model organism to study molecular mechanisms of membrane trafficking and cytoskeleton.

Intracompartmental sorting of essential myosin light chains: molecular dissection and in vivo monitoring by epitope tagging

The isoprotein-specific intracompartmental sorting of the three essential myosin light chains (LCs), the skeletal muscle LC-1f and LC-3f and the nonmuscle LC-3nm, was investigated. Epitope tagging was used to monitor the intracellular localization to different cytoskeletal structures of the exogenously introduced constructs in adult rat cardiomyocytes (ARCs), which exhibit both stress fibers and regenerating myofibrils. LC-1f and LC-3f bind almost exclusively to the sarcomeric myosin heavy chain (MHC) with high affinity, while the LC-3nm interacts with stress fibers and sarcomeres equally well. Sorting appears to be directed by a hierarchical order of different affinities. Domain mapping by deletion and by construction of a LC-1f/3nm chimera suggests that the LCs are composed of three functionally distinct domains: a basal MHC binding site in the C-terminus; the central part, modulating the preferential interaction with MHC isoforms; and the isoprotein-specific N-terminus of the essential LC, which is probably not involved in the sorting process.

Toxoplasma gondii myosins B/C: one gene, two tails, two localizations, and a role in parasite division

In apicomplexan parasites, actin-disrupting drugs and the inhibitor of myosin heavy chain ATPase, 2,3-butanedione monoxime, have been shown to interfere with host cell invasion by inhibiting parasite gliding motility. We report here that the actomyosin system of Toxoplasma gondii also contributes to the process of cell division by ensuring accurate budding of daughter cells. T. gondii myosins B and C are encoded by alternatively spliced mRNAs and differ only in their COOH-terminal tails. MyoB and MyoC showed distinct subcellular localizations and dissimilar solubilities, which were conferred by their tails. MyoC is the first marker selectively concentrated at the anterior and posterior polar rings of the inner membrane complex, structures that play a key role in cell shape integrity during daughter cell biogenesis. When transiently expressed, MyoB, MyoC, as well as the common motor domain lacking the tail did not distribute evenly between daughter cells, suggesting some impairment in proper segregation. Stable overexpression of MyoB caused a significant defect in parasite cell division, leading to the formation of extensive residual bodies, a substantial delay in replication, and loss of acute virulence in mice. Altogether, these observations suggest that MyoB/C products play a role in proper daughter cell budding and separation.

Rab7 and Rab9 are recruited onto late endosomes by biochemically distinguishable processes

Rab GTPases are localized to the surfaces of distinct membrane-bound organelles and function in transport vesicle docking and/or fusion. Prenylated Rab9, bound to GDP dissociation inhibitor-alpha, can be recruited selectively onto a membrane fraction enriched in late endosomes; this process is accompanied by nucleotide exchange. We used this system to address whether each Rab uses a distinct machinery to associate with its cognate organelle. Purified, prenylated Rab1B, Rab7, and Rab9 proteins were each reconstituted as stoichiometric complexes with purified GDP dissociation inhibitor-alpha, and their recruitment onto endosome- or ER-enriched membrane fractions was quantified. The two late endosomal proteins, Rab9 and Rab7, were each recruited onto endosome membranes with approximate apparent Km values of 9 and 22 nM, respectively. However, while control Rab9.GDP dissociation inhibitor-alpha complexes inhibited the initial rate of myc-tagged Rab9 recruitment with an apparent Ki of approximately 9 nM, Rab7 complexes inhibited this process much less effectively (apparent Ki approximately 112 nM). Similarly, complexes of the endoplasmic reticulum-localized Rab1B protein were even less potent than Rab7 complexes (apparent Ki approximately 405 nm). Rab9 complexes inhibited Rab7 recruitment with the same low efficacy as Rab7 complexes inhibited Rab9 recruitment. These experiments distinguish, biochemically, the recruitment of different Rab proteins onto a single class of organelle. Since Rab7 and Rab9 are both localized at least in large part, to late endosomes, this suggests that a single organelle may bear multiple Rab recruitment machines.

Dictyostelium myosin IK is involved in the maintenance of cortical tension and affects motility and phagocytosis

Dictyostelium discoideum myosin Ik (MyoK) is a novel type of myosin distinguished by a remarkable architecture. MyoK is related to class I myosins but lacks a cargo-binding tail domain and carries an insertion in a surface loop suggested to modulate motor velocity. This insertion shows similarity to a secondary actin-binding site present in the tail of some class I myosins, and indeed a GST-loop construct binds actin. Probably as a consequence, binding of MyoK to actin was not only ATP- but also salt-dependent. Moreover, as both binding sites reside within its motor domain and carry potential sites of regulation, MyoK might represent a new form of actin crosslinker. MyoK was distributed in the cytoplasm with a significant enrichment in dynamic regions of the cortex. Absence of MyoK resulted in a drop of cortical tension whereas overexpression led to significantly increased tension. Absence and overexpression of MyoK dramatically affected the cortical actin cytoskeleton and resulted in reduced initial rates of phagocytosis. Cells lacking MyoK showed excessive ruffling, mostly in the form of large lamellipodia, accompanied by a thicker basal actin cortex. At early stages of development, aggregation of myoK null cells was slowed due to reduced motility. Altogether, the data indicate a distinctive role for MyoK in the maintenance and dynamics of the cell cortex.

Myosin-I nomenclature

We suggest that the vertebrate myosin-I field adopt a common nomenclature system based on the names adopted by the Human Genome Organization (HUGO). At present, the myosin-I nomenclature is very confusing; not only are several systems in use, but several different genes have been given the same name. Despite their faults, we believe that the names adopted by the HUGO nomenclature group for genome annotation are the best compromise, and we recommend universal adoption.

Phosphorylation of chicken brain-type creatine kinase affects a physiologically important kinetic parameter and gives rise to protein microheterogeneity in vivo

In addition to the two monomer subunits of chicken brain-type creatine kinase (B-CK, EC, 2.7.3.2), termed Bb (basic) and Ba (acidic), another subspecies called Bb* was identified by chromatofocussing in the presence of 8 M urea (Quest et al., ). The latter low abundance protein species, isolated from tissue extracts, comigrated on 2D-gels with three minor species (Bb1-3), initially identified in immunoprecipitated, [35S]methionine labeled in vitro translation products of cDNA coding for the basic monomer Bb. During in vitro translation experiments in the presence of [32P]-gamma-ATP, Bb1-3 were labeled while phosphatase treatment eliminated these minor species. It is concluded that Bb* is identical to Bb1-3 and represents phosphorylated derivatives of Bb. B-CK dimer populations from different tissues were separated by ion-exchange chromatography and the Km values of the resulting fractions were determined under phospho-creatine (CP)-limiting conditions. In fractions containing only Bb and Bb* two kinetically different enzyme species were detected (Km values for CP = 1.6 mM and 0.8 mM), while fractions containing B-CK dimers composed of the major Ba and Bb monomers, but no Bb*, were homogeneous in this respect (Km for CP = 1.6 mM). Phosphorylation of Bb to yield Bb* is concluded to reduce the Km of B-CK dimers for CP by about 50%. This Km shift is within the range of CP concentrations found in tissues expressing the B-CK isoform and may therefore be of physiological relevance.

Dictyostelium discoideum protein disulfide isomerase, an endoplasmic reticulum resident enzyme lacking a KDEL-type retrieval signal

The primary activity of protein disulfide isomerase (PDI), a multifunctional resident of the endoplasmic reticulum (ER), is the isomerization of disulfide bridges during protein folding. We isolated a cDNA encoding Dictyostelium discoideum PDI (Dd-PDI). Phylogenetic analyses and basic biochemical properties indicate that it belongs to a subfamily called P5, many members of which differ from the classical PDIs in many respects. They lack an intervening inactive thioredoxin module, a C-terminal acidic domain involved in Ca2+ binding and a KDEL-type retrieval signal. Despite the absence of this motif, the ER is the steady-state location of Dd-PDI, suggesting the existence of an alternative retention mechanism for P5-related enzymes.

Identification of a novel saturable endoplasmic reticulum localization mechanism mediated by the C-terminus of a Dictyostelium protein disulfide isomerase

Localization of soluble endoplasmic reticulum (ER) resident proteins is likely achieved by the complementary action of retrieval and retention mechanisms. Whereas the machinery involving the H/KDEL and related retrieval signals in targeting escapees back to the ER is well characterized, other mechanisms including retention are still poorly understood. We have identified a protein disulfide isomerase (Dd-PDI) lacking the HDEL retrieval signal normally found at the C terminus of ER residents in Dictyostelium discoideum. Here we demonstrate that its 57 residue C-terminal domain is necessary for intracellular retention of Dd-PDI and sufficient to localize a green fluorescent protein (GFP) chimera to the ER, especially to the nuclear envelope. Dd-PDI and GFP-PDI57 are recovered in similar cation-dependent complexes. The overexpression of GFP-PDI57 leads to disruption of endogenous PDI complexes and induces the secretion of PDI, whereas overexpression of a GFP-HDEL chimera induces the secretion of endogenous calreticulin, revealing the presence of two independent and saturable mechanisms. Finally, low-level expression of Dd-PDI but not of PDI truncated of its 57 C-terminal residues complements the otherwise lethal yeast TRG1/PDI1 null mutation, demonstrating functional disulfide isomerase activity and ER localization. Altogether, these results indicate that the PDI57 peptide contains ER localization determinants recognized by a conserved machinery present in D. discoideum and Saccharomyces cerevisiae.

The tail domain of myosin M catalyses nucleotide exchange on Rac1 GTPases and can induce actin-driven surface protrusions

Members of the myosin superfamily play crucial roles in cellular processes including management of the cortical cytoskeleton, organelle transport and signal transduction. GTPases of the Rho family act as key control elements in the reorganization of the actin cytoskeleton in response to growth factors, and other functions such as membrane trafficking, transcriptional regulation, growth control and development. Here, we describe a novel unconventional myosin from Dictyostelium discoideum, MyoM. Primary sequence analysis revealed that it has the appearance of a natural chimera between a myosin motor domain and a guanine nucleotide exchange factor (GEF) domain for Rho GTPases. The functionality of both domains was established. Binding of the motor domain to F-actin was ATP-dependent and potentially regulated by phosphorylation. The GEF domain displayed selective activity on Rac1-related GTPases. Overexpression, rather than absence of MyoM, affected the cell morphology and viability. Particularly in response to hypo-osmotic stress, cells overexpressing the MyoM tail domain extended massive actin-driven protrusions. The GEF was enriched at the tip of growing protuberances, probably through its pleckstrin homology domain. MyoM is the first unconventional myosin containing an active Rac-GEF domain, suggesting a role at the interface of Rac-mediated signal transduction and remodeling of the actin cytoskeleton.

Extended lymph node dissection for gastric cancer: results of a prospective, multi-centre analysis of morbidity and mortality in 118 consecutive cases

This study reports interim data on post-operative morbidity, hospital mortality and duration of hospital stay of Italian patients undergoing extended lymph-node dissection combined with a pancreas-preserving technique for gastric cancer. Of the 218 patients admitted to one of eight general and/or university hospitals in North Italy, 118 were enrolled in the trial. Eligible patients presented with proven primary adenocarcinoma of the stomach without clinical evidence of distant, peritoneal and/or liver metastasis, or metastasis in para-aortic and retropancreatic nodes at intraoperative biopsy. Patients underwent the extended procedure as described by the Japanese Research Society for the Study of Gastric Cancer, following the Maruyama pancreas-preserving technique. A strict quality control system was used to ensure the performance of a standard surgical treatment. A surgeon of the reference centre (M.D.), who stayed at the National Cancer Center Hospital in Tokyo to learn the D2 technique from a specialist Japanese surgeon, became the trial supervisor and assisted each surgeon in all the Italian participating centres. The patients were staged according both to the TNM system and to the General Rules for the Gastric Cancer Study in Surgery and Pathology. Post-operative surgical complications developed in 21 patients (17.8%). The non-surgical complication rate was 2.5%. Reoperation was necessary in six patients (5%), all of whom survived. The 30-day mortality rate for the eligible group was 2.5%. The overall hospital mortality was the same. Total gastrectomy was associated with a slightly higher operative mortality (4.5% vs 1.3%). Only one patient died from an anastomotic leak. The rate of leakages was higher after total than after distal gastrectomy (15.9 vs 5.4%); the association of splenectomy and pancreatectomy worsened the morbidity rate. D2 lymphadenectomy with pancreas-preserving technique, when performed at experienced centres, seems a feasible and safe technique for the radical treatment of gastric cancer in selected Western patients.

How many is enough? Exploring the myosin repertoire in the model eukaryote Dictyostelium discoideum

The cytoplasm of eukaryotic cells is a very complex milieu and unraveling how its unique cytoarchitecture is achieved and maintained is a central theme in modern cell biology. It is crucial to understand how organelles and macro-complexes of RNA and/or proteins are transported to and/or maintained at their specific cellular locations. The importance of filamentous-actin-directed myosin-powered cargo transport was only recently realized, and after an initial explosion in the identification of new molecules, the field is now concentrating on their functional dissection. Direct connections of myosins to a variety of cellular tasks are now slowly emerging, such as in cytokinesis, phagocytosis, endocytosis, polarized secretion and exocytosis, axonal transport, etc. Unconventional myosins have been identified in a wide variety of organisms, making the presence of actin and myosins a hallmark of eukaryotism. The genome of S. cerevisiae encodes only five myosins, whereas a mammalian cell has the capacity to express between two and three dozen myosins. Why is it so crucial to arrive at this final census? The main questions that we would like to discuss are the following. How many distinct myosin-powered functions are carried out in a typical higher eukaryote? Or, in other words, what is the minimal set of myosins essential to accomplish the multitude of tasks related to motility and intracellular dynamics in a multicellular organism? And also, as a corollary, what is the degree of functional redundancy inside a given myosin class? In that respect, the choice of a model organism suitable for such an investigation is more crucial than ever. Here we argue that Dictyostelium discoideum is affirming its position as an ideal system of intermediate complexity to study myosin-powered trafficking and is or will soon become the second eukaryote for which complete knowledge of the whole repertoire of myosins is available.

Pure alveolar rhabdomyosarcoma of the corpus uteri: description of a case with increased serum level of CA-125

We report a morphological and immunohistochemical study of a case of pure alveolar rhabdomyosarcoma of the uterus in an 80-year-old woman. The diagnostic clues were the characteristic "alveolar" pattern of growth, the evidence of cross-striations in strap or elongated cells with abundant eosinophilic cytoplasms, the presence of multinucleated cells with peripherally placed "wreathlike" nuclei, and the expression of muscular antigens by the tumor cells. A thorough sampling of the tumor excluded areas of other types of heterologous or homologous sarcomas or the presence of coexisting adenoma or carcinoma. The other immunohistochemical data showed a high proliferative rate as well as a high rate of p53 overexpression in the small poorly differentiated rhabdomyoblasts. Interestingly, the large differentiated rhabdomyoblasts expressed CA-125, the antigenic determinant of nonmucinous epithelial ovarian tumors. The clinical course was very aggressive: the patient died 5 months after surgery because of disease progression. The pertinent literature is discussed.