Intermediate filament proteins with nuclear functions: NuMA, lamin-like proteins and MFP1.

Abstract

The -helical coiled coil is one of the principal subunit oligomerisation motifs in proteins, consisting of two or more -helices that twist around one another to form a supercoil. Coiled-coil proteins can be classified into two general categories. The first one comprises short coiledcoil domains called leucine zippers that form homoand hetero-dimers, as those found in transcription factors. The second class contains long coiled-coil domains with high polymerisation potential, which form a central rod configuration. The latter constitutes the intermediate filament (IF) protein family. The IF proteins are involved in structural organisation of cells, acting as scaffolds, fixing other proteins to solid state cell components, and also as cytoskeletal motors. They are also involved in molecular recognition and signal transduction (Burkhard et al., 2001). The IF proteins constitute a family of more than 40 different molecular species, classified into six main types (Coulombe et al., 2001); the group V corresponds to the lamins, nuclear proteins that play a role in nuclear organisation and replication (Schirmer et al., 2001). Some IFs associate with centromeres, nuclear matrix (NM), chromatin and the nuclear pore complex, and have DNA binding activity, histone affinity and similarity to NM proteins (Traub and Shoeman, 1994). In contrast to animals, only a reduced number of IFs have been identified in plants. Our previous ultrastructural studies had demonstrated a nucleoskeleton in plants containing both an organised structure underlying the nuclear envelope or lamina (Fig. 1A) (Moreno Diaz De La Espina, 1995) and an internal nucleoskeleton made up from branched knobbed 15–25 nm filaments, with a periodic organisation similar to those of the animal nucleoskeleton, as well as associated aggregates of different sizes (Fig. 1B,C) (Yu and Moreno Diaz De La Espina, 1999). Although from the 1990s we have made some contributions to the identification of IF and coiled-coil proteins in the plant matrix, such as lamins, NuMA and MFP1 (Fig. 1D–F), the molecular composition of the core filaments remains unknown. Nuclear mitotic apparatus protein (NuMA) is a 200– 240 kD coiled-coil protein first identified in animals. NuMA is a prominent component of nuclear core filaments. During mitosis, it relocates to the mitotic spindle and associates with motor proteins, contributing to microtubule stabilisation and spindle organisation. Although its functions during mitosis are well established, studies on its nuclear function are scarce (Taimen et al., 2000). Using an immunological approach, three onion homologues of NuMA, of 210, 220 and 230 kD (Fig. 1D), were identified in the filaments of its nucleoskeleton (Fig. 1G) and in the mitotic spindle (Yu and Moreno Diaz De La Espina, 1999). The potential of NuMA oligomers to form higher order structures in animals (Harborth et al., 1999) and the localisation of AcNuMA in core filaments, suggest its structural role in plant nuclear organisation. Lamin-like proteins with conserved Mr and pI values, antigenically related to animal A and B lamins, were detected in the lamina and the internal NM of several plants (Fig. 1E,H) (see Moreno Diaz De La Espina, 1995 for a review). In spite of all their similarities and of the important nuclear functions played by lamins (Schirmer et al., 2001), lamin genes have not been identified in plants as yet, suggesting that they could have distinctly related nucleoskeletal proteins that functionally replace lamins. Recently, Gindullis et al. (2002) reported a novel family of long coiled-coil proteins from plants, which represents a good candidate for the abovementioned fact. * Corresponding author. Tel.: +34-91-561-1800; fax: +34-91-562-7518. E-mail address: smoreno@cib.csic.es (S. Moreno Diaz De La Espina). Cell Biology International 27 (2003) 233–235 Cell Biology International