Articulins and epiplasmins: two distinct classes of cytoskeletal proteins of the membrane skeleton in protists

The cortex of ciliates, dinoflagellates and euglenoids comprises a unique structure called the epiplasm, implicated in pattern-forming processes of the cell cortex and in maintaining cell shape. Despite significant variation in the structural organization of their epiplasm and cortex, a novel type of cytoskeletal protein named articulin is the principal constituent of the epiplasm in the euglenoid Euglena and the ciliate Pseudomicrothorax. For another ciliate, Paramecium, epiplasmins, a group of polypeptides with common biochemical properties, are the major constituents of the epiplasm. Using molecular tools and affinity purification we have selected polyclonal antibodies and identified epitopes of monoclonal antibodies that identify epitopes characteristic of articulins and epiplasmins. With these antibodies we have analysed the occurrence of the two types of cytoskeletal proteins in a dinoflagellate, a euglenoid and several ciliates. Our results indicate that both articulins and epiplasmins are present in these organisms, suggesting that both contribute to the organization of the membrane skeleton in protists. Articulins and epiplasmins represent two distinct classes of cytoskeletal proteins, since different polypeptides were labeled by articulin core domain-specific or epiplasmin epitope-specific antibodies in each organism studied. In one case, a polypeptide in Pseudomicrothorax was identified that reacts with both articulin core domain-specific and with anti-epiplasmin monoclonal antibodies; however, the epiplasmin monoclonal antibody epitope was mapped to the C terminus of the polypeptide, well outside the central VPV-repeat core domain that contains the articulin monoclonal antibody epitope and that is the hallmark of the articulins.

Characterisation of two articulins, the major epiplasmic proteins comprising the membrane skeleton of the ciliate Pseudomicrothorax

Most protists possess a unique membrane skeleton, the epiplasm, which is involved in pattern forming processes of the cell cortex and functions in maintaining cell shape. Articulins, a novel class of cytoskeletal proteins, are major constituents of the epiplasm. We have isolated cDNAs encoding the two major articulins of the ciliate Pseudomicrothorax dubius. Peptide sequence data confirm the identity of the cloned cDNAs encoding articulins 1 and 4. With the data presented here sequence information for all major articulins of ciliates as well as the distantly related euglenoids is available. Sequence comparison of the two newly characterised ciliate articulins with the previously determined sequences of p60, a minor articulin of the same species, and the two euglenoid articulins reveals general sequence principles and uncovers new features of this protein family. The hallmark of articulins is a central core domain of repetitive motifs of alternating valine and proline residues, the VPV-motif. These VPV-motif repeats are either 12-residues, or in some places, six residues long. Positively and negatively charged residues segregate in register with valine and proline positions. The VPV-motif is unique to articulins. The terminal domains flanking the core are generally hydrophobic and contain a series of hexa- or heptapeptide repeats rich in glycine and hydrophobic residues. The sequences of these short repeats are very similar in articulins of the same species but are not conserved between euglenoids and ciliates.

Purification, in vitro reassembly, and preliminary sequence analysis of epiplasmins, the major constituent of the membrane skeleton of Paramecium

The epiplasmic layer, a continuous rigid granulo-fibrillar sheet directly subtending the surface membranes of Paramecium, is one of the outermost of the various cytoskeletal networks that compose it cortex. We have previously shown that the epiplasm consists of a set of 30 to 50 protein bands on SDS-PAGE in the range 50 to 33 kDa, the epiplasmins. We report a purification procedure for the set of epiplasmic proteins, a description of their physicochemical and reassembly properties, and a preliminary characterization of their sequence. The conditions for solubilization of the epiplasm and for in vitro reassembly of its purified constituents ar described. Reassembly of the entire set of proteins and of some (but not all) subsets are shown to yield filamentous aggregates. Microsequences of two purified bands of epiplasmins reveal a striking amino acid sequence consisting of heptad repeats of only three main amino acids, P, V, and Q. These repeats were confirmed by DNA sequencing of polymerase chain reaction products. The motif is QPVQ-h, in which h is a hydrophobic residue. This may constitute the core of the epiplasmin sequence and, in view of the tendency of such a sequence to form a coiled-coil, may account for the remarkable self-aggregation properties of epiplasmins.

Cortical structure and function in euglenoids with reference to trypanosomes, ciliates, and dinoflagellates

The membrane skeletal complex (cortex) of euglenoids generates and maintains cell form. In this review we summarize structural, biochemical, physiological, and molecular studies on the euglenoid membrane skeleton, focusing specifically on four principal components: the plasma membrane, a submembrane layer (epiplasm), cisternae of the endoplasmic reticulum, and microtubules. The data from euglenoids are compared with findings from representative organisms of three other protist groups: the trypanosomes, ciliates, and dinoflagellates. Although there are significant differences in cell form and phylogenetic affinities among these groups, there are also many similarities in the organization and possibly the function of their cortical components. For example, an epiplasmic (membrane skeletal) layer is widely used for adding strength and rigidity to the cell surface. The ER/alveolus/amphiesmal vesicle may function in calcium storage and regulation, and in mediating assembly of surface plates. GPI-linked variable surface antigens are characteristic of both ciliates and the unrelated trypanosomatids. Microtubules are ubiquitous, and cortices in trypanosomes may relay exclusively on microtubules and microtubule-associated proteins for maintaining cell form. Also, in agreement with previous suggestions, there is an apparent preservation of many cortical structures during cell duplication. In three of the four groups there is convincing evidence that part or all of the parental cortex persists during cytokinesis, thereby producing mosaics or chimeras consisting of both inherited and newly synthesized cortical components.

Identification of the epiplasmins, a new set of cortical proteins of the membrane cytoskeleton in Paramecium

In most ciliates, the epiplasm, a superficial cytoskeletal layer of variable thickness, both surrounds basal bodies and interacts tightly with adjacent membrane networks; it constitutes the predominant structure in Paramecium cell ghosts. Previous indirect data suggested several cortical proteins as potential constituents of the epiplasm. New sharp monoclonal antibodies presented in this paper, positive both on immunotransfers and in immunocytochemical tests carried out on permeabilized cells and ultrathin sections, definitively identify the epiplasmins: a set of about twenty protein bands ranging from 45 to 33 kDa and making up the bulk of the epiplasmic layer. The complete epiplasmin pattern characterized from gradient-purified cortex is also present in unfractionated whole cells, confirming that the pattern is not generated artifactually. Comparative one-step extractions, performed either in 1 M KI or in 4 M urea, solubilize the epiplasmins as a whole, indicating that all of them share very similar biochemical properties. Two-dimensional electrophoresis shows the great complexity of this epiplasmin group. Epiplasmin solubilization properties are discussed with respect to other models of membrane-cytoskeleton interaction developed among protists and metazoans and also to intermediate filaments, specially lamins. Immunofluorescent labelling combined with confocal microscopy permits a more detailed study of epiplasm formation at the level of the fission furrow, with new insights into two successive steps of epiplasm growth. A first series of interspecific reactions has been carried out with one of the anti- epiplasmin antibodies, yielding results which are discussed in an evolutionary framework.