The CaaX motif is required for isoprenylation, carboxyl methylation, and nuclear membrane association of lamin B2

Nucleoplasmic localization of prelamin A: implications for prenylation-dependent lamin A assembly into the nuclear lamina

The synthesis of the nuclear lamina protein lamin A requires the prenylation-dependent processing of its precursor protein, prelamin A. Unlike p21ras, which undergoes similar initial posttranslational modifications, maturation of lamin A results in the proteolytic removal of the prenylated portion of the molecule. We have used an in vitro prenylation system to demonstrate the nature of the prenyl substituent on prelamin A to be a farnesyl group. Further, the in vitro farnesylation of prelamin A requires an intact cysteine-aliphatic-aliphatic-other (CAAX) amino acid sequence motif at its carboxyl terminus. The effect of blocking the prenylation of prelamin A on its localization and assembly into the nuclear lamina was investigated by indirect immunofluorescence. Expression of wild-type prelamin A in lovastatin-treated cells showed that nonprenylated prelamin A accumulated as nucleoplasmic particles. Upon addition of mevalonate to lovastatin-treated cells, the wild-type lamin A was incorporated into the lamina within 3 hr. Expression of a mutant lamin A in which the carboxyl-terminal 21 amino acids were deleted resulted in a lamin molecule that was directly assembled into the lamina. These results indicate that the carboxyl-terminal peptide of prelamin A blocks its proper assembly into the nuclear lamina and that the prenylation-initiated removal of this peptide can occur in the nucleus.

Mitogen-activated protein kinases phosphorylate nuclear lamins and display sequence specificity overlapping that of mitotic protein kinase p34cdc2

Members of the mitogen-activated protein (MAP) kinase family are implicated in mediating entry of cells into the cell cycle, as well as passage through meiotic M phase. These kinases have attracted much interest because their activation involves phosphorylation on both tyrosine and threonine residues, but little is known about their physiological targets. In this study, two distinct members of the MAP kinase family (p44mpk and p42mapk) are shown to phosphorylate chicken lamin B2 at a single site identified as Ser16. Moreover, these MAP kinases cause depolymerization of in-vitro-assembled longitudinal lamin head-to-tail polymers. Ser16 was previously shown to be phosphorylated during mitosis in vivo, and to be a target of the mitotic protein kinase p34cdc2 in vitro. Accordingly, lamins were proposed to be direct in vivo substrates of p34cdc2. This proposal is supported by quantitative analyses indicating that lamin B2, when assayed in vitro, is a substantially better substrate for p34cdc2 than for MAP kinases. Nevertheless, a physiological role of MAP kinases in lamin phosphorylation is not excluded. The observation that members of the MAP kinase family display sequence specificities overlapping that of p34cdc2 raises the possibility that some of the purported substrates of p34cdc2 may actually be physiological substrates of MAP kinases.

Assembly-disassembly of the nuclear lamina

Recently, progress in the study of lamins has been made in three areas: signals required for targetting newly synthesized lamins to the correct subnuclear compartment have been identified; information on lamina assembly has been obtained from in vitro studies using bacterially expressed proteins; and a mechanistic explanation for how the nuclear lamina is diassembled at the onset of mitosis is emerging.

Interaction of Xenopus lamins A and LII with chromatin in vitro mediated by a sequence element in the carboxyterminal domain

Morphological data suggest an interaction of the nuclear lamina with chromatin which markedly changes during the cell cycle. To study the molecular basis of this interaction we developed a novel lamin/chromatin binding assay that quantitated the binding of soluble, radiolabeled lamins to minichromosomes assembled in Xenopus laevis oocyte nuclear extracts. Lamins were derived from couple in vitro transcription and translation of the corresponding cDNAs. Chromatin binding was detected by monitoring the cofractionation with assembled minichromosomes in gel filtration and sucrose gradient centrifugation. Binding of lamins to chromatin increased with chromatin concentration and was accompanied by lamin polymerization. Lamins of the A-(Xenopus LA and human LC) as well as the B-type (Xenopus LI and LII) showed strikingly different chromatin binding capacities. Lamins A and LII bound efficiently of lamins LI and LC was detected. Using site-directed mutagenesis, we were able to define carboxy-terminal sequence elements of LA and LII required for the observed lamin/chromatin interaction that are rich in serine, threonine, and glycine residues. Competition experiments with a synthetic peptide containing the chromatin binding motif of lamin A corroborate the importance of these sequence elements in the lamin/chromatin interaction.

Characterization of A 54-kD protein of the inner nuclear membrane: evidence for cell cycle-dependent interaction with the nuclear lamina

Using a mAb (R-7), we have characterized a 54-kD protein of the chicken nuclear envelope. Based on its biochemical properties and subnuclear distribution p54 is likely to be an integral membrane component specific to the inner nuclear membrane. Fractionation experiments indicate that p54 interacts, directly or indirectly, with the nuclear lamina, and analysis of p54 in cultured cells suggests that this interaction is controlled by cell cycle-dependent posttranslational modification, most likely phosphorylation. Modification of p54 results in a slightly reduced electrophoretic mobility, and it converts the protein from a detergent-resistant to a detergent-extractable form. Detergent solubilization of p54 can be induced in vivo by treating isolated nuclei or nuclear envelopes with highly purified cdc2 kinase, one of the most prominent kinases active in mitotic cells. These results suggest that mitotic phosphorylation of p54 might contribute to control nuclear envelope dynamics during mitosis in vivo.

Disassembly of in vitro formed lamin head-to-tail polymers by CDC2 kinase

The nuclear lamina is an intermediate filament-type network underlying the inner nuclear membrane. At the onset of mitosis it depolymerizes, presumably in response to phosphorylation of the lamin proteins. Recently, cdc2 kinase, a major regulator of the eukaryotic cell cycle, was shown to induce lamina depolymerization when incubated with isolated nuclei. Here, we have analysed the structural consequences of lamin phosphorylation by cdc2 kinase using lamin head-to-tail polymers reconstituted in vitro from bacterially expressed chicken lamin B2 protein as a substrate. The effects of phosphorylation were monitored by both a pelleting assay and electron microscopy. We show that lamin B2 head-to-tail polymers disassemble in response to phosphorylation of specific sites that are phosphorylated also during mitosis in vivo. These sites are located within SP/TP motifs N- and C-terminal to the central alpha-helical rod domain of lamin proteins. Subsequent dephosphorylation of these sites by purified phosphatase 1 allows reformation of lamin head-to-tail polymers. The relative importance of N- and C-terminal phosphorylation sites for controlling the assembly state of nuclear lamins was assessed by mutational analysis. Polymers formed of lamin proteins carrying mutations in the C-terminal phosphoacceptor motif could still be disassembled by cdc2 kinase. In contrast, a single point mutation in the N-terminal site (Ser16----Ala) rendered head-to-tail polymers resistant to disassembly. These results emphasize the importance of the N-terminal end domain for lamin head-to-tail polymerization in vitro, and they demonstrate that phosphorylation-dephosphorylation is sufficient to control the longitudinal assembly of lamin B2 dimers.