INTRODUCTION The complex family of intermediate filament (IF) proteins in vertebrates comprises more than 40 different proteins of variable molecular mass and specific expression, classified into five families. Even though they are functionally very different, all of them share a very conserved molecular structure with a central α-helix rod domain with typical coiled-coil segments, however the N-terminal heads and C- terminal tails are very variable (Osborn and Weber, 1986; Franke, 1987) and form homodimeric complexes. All the IFs tested contain a common epitope at the C-terminal end of the helical rod domain, which is recognized by a mon- oclonal antibody named IFA (Pruss et al., 1981). It is now known that IF proteins are not exclusive to vertebrates, and they have been characterized in several invertebrates (Dodemont et al., 1990; Riemer et al., 1991). Although IFs have not been described as such in plants, the IFA antigen appears to be widely distributed in them (Dawson et al., 1985; Goodbody et al., 1989; Frederick, et al., 1992; Li and Roux, 1992), and IF epitopes have been reported in the cytoskeletons of several plant species (see Shaw et al., 1991). In addition, crossreactivity has been demonstrated between plant and animal IFs. The data avail- able suggest that the epitopes detected on plants are generic and conserved phylogenetically (Shaw et al., 1991). The information on plant IFs is scarce. Plant cells do not have an organized cytoskeletal network of IFs, although the pres- ence of IF antigens in both fibrillar bundles (FBs), which are capable of forming 10 nm filaments after denaturation, and microtubule-associated arrays have been demonstrated (Goodbody et al., 1989; Hargreaves et al., 1989b). The nuclear lamina is a scaffolding structure that corre- sponds to the residual elements of the nuclear envelope, which has been, so far, described in eukaryotes ranging from Protozoa to vertebrates, as being a universal compo- nent of eukaryotic nuclei (Gerace, 1986; Krohne and Benavente, 1986; Doring and Stick, 1990). The lamina is made up basically of a network of lamin filaments with which the residual elements of the pore-complexes are associated (Krohne and Benavente, 1986; Aebi et al., 1984), and shows continuity with the cytoskeleton of inter- mediate filaments (Capco et al., 1984; Carmo-Fonseca et al., 1987). The major structural proteins of the lamina are the lamins, a multigenic family of proteins that share many structural features with the IF proteins and are classified 431 Journal of Cell Science 106, 431-439 (1993) Printed in Great Britain © The Company of Biologists Limited 1993 We have used polyclonal and monoclonal antibodies against different lamins from vertebrates, and the IFA antibody recognizing all kinds of intermediate filament proteins, to investigate the lamins of the nuclear matrix of Allium cepa meristematic root cells. All the antibod- ies react in the onion nuclear matrix with bands in the range of 60-65 kDa, which are enriched in the nuclear matrix after urea extraction, and do not crossreact with other antibodies recognizing intermediate filaments in plants (AFB, anti-vimentin and MAC 322), ruling out crossreaction with contaminating intermediate filaments of cytoplasmic bundles. In 2-D blots the chicken anti- lamin serum reacts with one spot at 65 kDa and pI 6.8 and the anti B-type lamin antibodies with another one at 64 kDa and pI 5.75. Both crossreact with IFA. The lamin is localized at the nuclear periphery and the lamina by indirect immunofluorescence. Immuno- gold labelling of nuclear matrix sections reveals that the protein is not only associated with the lamina, but also with the internal matrix. Taken together these results reveal that higher plants, which do not possess an orga- nized network of cytoplasmic intermediate filaments, nevertheless present a well-organized lamina containing lamins in which at least one of them is immunologically related to vertebrate lamin B. Our data confirm that lamins are very old members of the intermediate fila- ment proteins that have been better conserved in plants during evolution than their cytoplasmic counterparts. Key words: higher plants, lamins, immunoblotting, immunofluorescence SUMMARY Immunological characterization of lamins in the nuclear matrix of onion cells A. Mínguez and S. Moreno Díaz de la Espina* Laboratorio de Biología Celular y Molecular Vegetal, Centro de Investigaciones Biológicas, Velázquez, 144. E-28006 Madrid, Spain *Author for correspondence