The nuclear envelope defines the barrier between the nucleus and cytoplasm and features inner and outer membranes separated by a perinuclear space (PNS). The inner nuclear membrane contains specific integral proteins that include Sun1 and Sun2. Although the outer nuclear membrane (ONM) is continuous with the endoplasmic reticulum, it is nevertheless enriched in several integral membrane proteins, including nesprin 2 Giant (nesp2G), an 800-kD protein featuring an NH2-terminal actin-binding domain. A recent study (Padmakumar, V.C., T. Libotte, W. Lu, H. Zaim, S. Abraham, A.A. Noegel, J. Gotzmann, R. Foisner, and I. Karakesisoglou. 2005. J. Cell Sci. 118:3419–3430) has shown that localization of nesp2G to the ONM is dependent upon an interaction with Sun1. In this study, we confirm and extend these results by demonstrating that both Sun1 and Sun2 contribute to nesp2G localization. Codepletion of both of these proteins in HeLa cells leads to the loss of ONM-associated nesp2G, as does overexpression of the Sun1 lumenal domain. Both treatments result in the expansion of the PNS. These data, together with those of Padmakumar et al. (2005), support a model in which Sun proteins tether nesprins in the ONM via interactions spanning the PNS. In this way, Sun proteins and nesprins form a complex that links the nucleoskeleton and cytoskeleton (the LINC complex).

During prophase in higher cells, centrosomes localize to deep invaginations in the nuclear envelope in a microtubule-dependent process. Loss of nuclear membranes in prometaphase commences in regions of the nuclear envelope that lie outside of these invaginations. Dynein and dynactin complex components concentrate on the nuclear envelope prior to any changes in nuclear envelope organization. These observations suggest a model in which dynein facilitates nuclear envelope breakdown by pulling nuclear membranes and associated proteins poleward along astral microtubules leading to nuclear membrane detachment. Support for this model is provided by the finding that interference with dynein function drastically alters nuclear membrane dynamics in prophase and prometaphase.

Abstract. Centromere protein-F (CENP-F) is a mammalian kinetochore protein that was recently identified by an autoimmune serum (Rattner,

Microtubule nucleation and organization by the centrosome require ␥-tubulin, a protein that exists in a macromolecular complex called the ␥-tubulin ring complex (␥TuRC). We report characterization of CDK5RAP2, a novel centrosomal protein whose mutations have been linked to autosomal recessive primary microcephaly. In somatic cells, CDK5RAP2 localizes throughout the pericentriolar material in all stages of the cell cycle. When overexpressed, CDK5RAP2 assembled a subset of centrosomal proteins including ␥-tubulin onto the centrosomes or under the microtubule-disrupting conditions into microtubule-nucleating clusters in the cytoplasm. CDK5RAP2 associates with the ␥TuRC via a short conserved sequence present in several related proteins found in a range of organisms from fungi to mammals. The binding of CDK5RAP2 is required for ␥TuRC attachment to the centrosome but not for ␥TuRC assembly. Perturbing CDK5RAP2 function delocalized ␥-tubulin from the centrosomes and inhibited centrosomal microtubule nucleation, thus leading to disorganization of interphase microtubule arrays and formation of anastral mitotic spindles. Together, CDK5RAP2 is a pericentriolar structural component that functions in ␥TuRC attachment and therefore in the microtubule organizing function of the centrosome. Our findings suggest that centrosome malfunction due to the CDK5RAP2 mutations may underlie autosomal recessive primary microcephaly. INTRODUCTIONIn animal cells, the centrosome is the primary microtubule (MT) organizing center (MTOC), which plays a key role in the control of the temporal and spatial distribution of MT networks (Ou and Rattner, 2004;Doxsey et al., 2005;Luders and Stearns, 2007). Typically, centrosomes are positioned at the focus of a radial array of MTs during interphase and are incorporated into spindle poles during mitosis. Interphase centrosomes are composed of a pair of centrioles embedded in a cloud of electron-dense pericentriolar material (PCM). The centriole has a well-defined structure with MT triplets arranged into a cylinder, whereas the organization of the PCM is less apparent. Early studies have shown the existence of a salt (2 M KI)-insoluble scaffold/matrix underlying the PCM (Moritz et al., 1998;Schnackenberg et al., 1998). This matrix, formed with a high content of large coiled-coil proteins, provides binding sites for the tubulin family member ␥-tubulin and other proteins associated with centrosomal functions, such as MT nucleation. Through the cell cycle, the PCM varies in volume and the MT-nucleating activity, which are smallest in G1-phase and biggest during mitosis. In addition, proteins have a precise and cell cycle-specific placement within the PCM that has been observed as a tubular structure located along the surface of the centriole and around its proximal end but not around its distal end (Ou and Rattner, 2000;Ou et al., 2003). The outer surface of the PCM is dynamic and PCM proteins found in the cytoplasm transit to the PCM either by diffusion or via MTs.Distributed throughout the PCM, ␥-tubulin ...

We report the interactions amongst 20 proteins that specify their assembly to the centromere–kinetochore complex in human cells. Centromere protein (CENP)-A is at the top of a hierarchy that directs three major pathways, which are specified by CENP-C, -I, and Aurora B. Each pathway consists of branches that intersect to form nodes that may coordinate the assembly process. Complementary EM studies found that the formation of kinetochore trilaminar plates depends on the CENP-I/NUF2 branch, whereas CENP-C and Aurora B affect the size, shape, and structural integrity of the plates. We found that hMis12 is not constitutively localized at kinetochores, and that it is not essential for recruiting CENP-I. Our studies also revealed that kinetochores in HeLa cells contain an excess of CENP-A, of which ∼10% is sufficient to promote the assembly of normal levels of kinetochore proteins. We elaborate on a previous model that suggested kinetochores are assembled from repetitive modules (Zinkowski, R.P., J. Meyne, and B.R. Brinkley. 1991. J. Cell Biol. 113:1091–110).