A new model for nuclear lamina organization.

Lamins are intermediate filament proteins that form a network lining the inner nuclear membrane. They provide mechanical strength to the nuclear envelope, but also appear to have many other functions as reflected in the array of diseases caused by lamin mutations. Unlike other intermediate filament proteins, they do not self-assemble into 10 nm filaments in vitro and their in vivo organization is uncertain. We have recently re-examined the organization of a simple B-type lamina in Xenopus oocytes [Goldberg, Huttenlauch, Hutchison and Stick (2008) J. Cell Sci. 121, 215-225] and shown that it consists of tightly packed 8-10 nm filaments with regular cross-connections, tightly opposed to the membrane. When lamin A is expressed in oocytes, it forms organized bundles on top of the B lamina. This has led to a new model for lamina organization which is discussed in the present paper.

Filaments made from A- and B-type lamins differ in structure and organization.

Lamins are intermediate filament proteins and the major component of the nuclear lamina. Current views of the lamina are based on the remarkably regular arrangement of lamin LIII in amphibian oocyte nuclei. We have re-examined the LIII lamina and propose a new interpretation of its organization. Rather than consisting of two perpendicular arrays of parallel filaments, we suggest that the oocyte lamina consists of parallel filaments that are interconnected in register to give the impression of a second set of perpendicular filaments. We have also used the oocyte system to investigate the organization of somatic lamins. Currently, it is not feasible to examine the organization of somatic lamins in situ because of their tight association with chromatin. It is also difficult to assemble vertebrate lamin filaments in vitro. Therefore, we have used the oocyte system, where exogenously expressed somatic B-type and A-type lamins assemble into filaments. Expression of B-type lamins induces the formation of intranuclear membranes that are covered by single filament layers. LIII filaments appear identical to the endogenous lamina, whereas lamin B2 assembles into filaments that are organized less precisely. Lamin A induces sheets of thicker filaments on the endogenous lamina and significantly increases the rigidity of the nuclear envelope.

Occurrence of articulins and epiplasmins 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. Articulins, a novel class of cytoskeletal proteins, are major constituents of the epiplasm in the flagellate Euglena gracilis and the ciliate Pseudomicrothorax dubius. The hallmark of articulins is a core domain of repetitive motifs of alternating valine and proline residues, the VPV-motif. The VPV-motif repeats are 12 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. Using molecular and immunological tools we show that articulins are also present in the dinoflagellate Amphidinium carterae and the ciliates Paramecium tetraurelia and Paramecium caudatum, Tetrahymena pyriformis, and Euplotes aediculatus. Our analysis further shows that epiplasmins, a group of epiplasmic proteins first characterized in Paramecium, are also present in all these species. Moreover, we present evidence that epiplasmins and articulins represent two distinct classes of cytoskeletal proteins.