Journal of Structural Biology 155 (2006) 340–350 www.elsevier.com/locate/yjsbi 1047-8477/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jsb.2006.03.026 Solubility properties and speciWc assembly pathways of the B-type lamin from Caenorhabditis elegans Nicole Foeger a,1 , Naama Wiesel b , Dorothee Lotsch a , Norbert Mücke c , Laurent Kreplak d , Ueli Aebi d , Yosef Gruenbaum b , Harald Herrmann a,¤ a Division of Molecular Genetics, German Cancer Research Center, Im Neuenheimer Feld 580, D-69120 Heidelberg, Germany b Department of Genetics, Institute of Life Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel c Division of Biophysics of Macromolecules, German Cancer Research Center, Im Neuenheimer Feld 580, D-69120 Heidelberg, Germany d M. E. Mueller Institute for Structural Biology, Biozentrum, University of Basel, Klingelbergerstrasse 70, CH-4056 Basel, Switzerland Received 20 February 2006; accepted 28 March 2006 Available online 27 April 2006 Abstract Lamins are nucleus-speciWc intermediate Wlament (IF) proteins that together with a complex set of membrane proteins form a Wlamen- tous meshwork tightly adhering to the inner nuclear membrane and being associated with the nuclear pore complexes. This so-called nuclear lamina provides mechanical stability and, in addition, has been implicated in the spatial organization of the heterochromatin. While increasing knowledge on the biological function of lamins has been obtained in recent years, the assembly mechanism of lamin Wla- ments at the molecular level has remained largely elusive. Therefore, we have now more systematically investigated lamin assembly in vitro. Using Caenorhabditis elegans lamin, which has been reported to assemble into 10-nm Wlaments under low ionic strength condi- tions, we investigated the assembly kinetics of this protein into Wlaments in more detail using both His-tagged and un-tagged recombinant proteins. In particular, we have characterized distinct intermediates in the Wlament assembly process by analytical ultracentrifugation, electron and atomic force microscopy. In contrast to the general view that lamins assemble only slowly into Wlaments, we show that in vitro association reactions are extremely fast, and depending on the ionic conditions employed, signiWcant Wlamentous assemblies form within seconds.  2006 Elsevier Inc. All rights reserved. Keywords: Analytical ultracentrifugation; Electron microscopy; Intermediate Wlament; Lamin assembly; Nucleus; Paracrystals 1. Introduction The nuclear lamina is a meshwork assembled from lamin Wlaments and lamin-binding proteins that is tightly adher- ing to the inner nuclear membrane. In the nuclear interior, lamins appear to be part of stable complexes, however, the structure and composition of these complexes are unknown (Foisner, 2001; Gruenbaum et al., 2005). Hence, the lamins comprise the major constituents of the lamina proper, and more generally, of the nuclear scaVold. Lamins are type V intermediate Wlament (IF) proteins, which are characterized by a short— typically 30 amino acid long—amino-terminal head domain, a long (i.e., 352 amino acids compared to 310 amino acids of vertebrate cytoplasmic IF proteins) central -helical coiled-coil rod domain, and a complex carboxy- terminal tail domain harboring a nuclear localization signal (NLS) and a globular Ig-like fold (Dhe-Paganon et al., 2002; Krimm et al., 2002; Strelkov et al., 2004). Whereas the central rod domain mediates lamin dimerization, the head domain and the carboxy-terminal end of the -helical rod domain mediate head-to-tail association and higher-order assembly of dimers (Stuurman et al., 1998). * Corresponding author. Fax: +49 6221 42 3519. E-mail address: h.herrmann@dkfz-heidelberg.de (H. Herrmann). 1 Present address: Max F. Perutz Laboratories, University Departments at the Vienna Biocenter, Department of Medical Biochemistry, Medical University of Vienna, Dr. Bohrgasse 9/3, 1030 Vienna, Austria.