Cell News 2/2014
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(CenA and CenB) of the amoebozoan Dictyostelium cannot be
assigned to either of the two subfamilies. However, both are
predominantly associated with the nucleus, with CenA (origi-
nally called DdCrp) being concentrated at centromeres and to
a lesser extent at the centrosome, and CenB inside the nuc-
leus (Daunderer et al., 2001; Mana-Capelli et al., 2009). While
the exact function of CenA is unknown, CenB is important for
nuclear architecture and centrosome attachment to the nuc-
leus. The latter function is also found in budding yeast, where
Cdc31p together with its binding partner Sfi1p forms the half
bridge, which tethers the nascent SPB to the nuclear envelope
(Li et al., 2006).
Centrosomal LINCs to the nuclear lamina
The nuclear envelope is underlaid by a morphologically distinct
lamina first described in an amoeba (Pappas, 1956). Yet, until
recently, the nuclear lamina was characterized on a molecular
level only in metazoans. Here its major components are type V
intermediate filaments called lamins. They have a size of 60-80
kDa and consist of an approx. 370 aa coiled coil domain pre-
ceded by a short head domain and followed by a tail domain.
The head domain possesses a CDK1 phosphorylation site at its
end, and the tail domain includes a basic nuclear localization
sequence, an Ig-fold domain and a CaaX-box at its end. The
latter is the target for C-terminal prenylation, which is requi-
red for proper assembly of a lamin filament network at the NE.
Recent studies have characterized nuclear lamina proteins in
Trypanosoma (discicristates), higher plants and Dictyostelium
(DuBois et al., 2012; Krüger et al., 2012; Ciska et al., 2013).
While NMCP (nuclear matrix constituent proteins) of higher
plants and NUP-1 in trypanosoma are clearly functionally ana-
log to lamins, they do not appear to be evolutionary related
to lamins or to each other. Although they are also long coiled
coil proteins they show no sequence similarity to lamins. In
contrast the NE81 protein found in four amoebozan species
so far (Dictyostelium discoideum, D. falciparum, D. purpureum,
Polysphondylium pallidum) is not only functionally analog to
lamins, it also shows clear sequence similarity to lamins, si-
milar regulation and post translational processing and should
thus be considered a bona fide lamin (Batsios et al., 2012; Krü-
ger et al., 2012). This pushed the evolutionary origin of lamins
from their first appearance in metazoans back to the origin of
unikonts or even earlier (Devos et al., 2014). It is rather likely
that LECA possessed a nuclear lamina, however, it is unclear
whether it was composed of NUP-1-like, NMCP-like or lamin-
like proteins. LECA could have had a complex lamina composed
of all three types of proteins and all except one protein family
could have been lost during evolution in different eukaryotic
groups. Alternatively, plants and discicristata may have repla-
ced a lamin-based lamina by NUP-1 or NMCPs, respectively.
By contrast to the composition of the lamina itself, nuclear
attachment of the centrosome to the lamina is an ancient fea-
ture, since it is found in almost all vegetative eukaryotic cells
possessing a clearly discernible centrosomal organelle. This is
in line with the hypothesis stated above that the precursor of
the centrosome has split into a plasma membrane associated
unit required for membrane architecture and locomotion and a
nuclear unit involved in mitotic spindle organization (Cavalier-
Smith, 2010). The idea that the latter had a dual centromere/
centrosome function is supported by the fact that both struc-
tures are still very closely associated with each other during
the entire cell cycle, as for example in fission yeast or Dicty-
ostelium where centromeres are clustered close to the inner
nuclear membrane and permanently associated with the SPB/
centrosome at the cytoplasmic face of the nucleus (Kaller et al.,
2006; King et al., 2008; Schulz et al., 2009). In Dictyostelium
this linkage involves a conserved set of proteins that is now
known for its role in centrosome attachment to the nucleus
in all major eukaryotic groups. It includes Sun-family proteins,
dynein and nuclear lamina proteins (Schulz et al., 2009). Sun
proteins are widespread in all major eukaryotic groups. In most
organisms they are concentrated at the inner nuclear membra-
ne and interact with proteins of the KASH-family in the outer
nuclear membrane forming a so-called LINC complex (Stewart-
Hutchinson et al., 2008; Starr and Fridolfsson, 2010; Rothballer
and Kutay, 2013). Different KASH proteins manage direct or
indirect connection to all three cytoskeletal filament systems
(microtubules, actin filaments, intermediate filaments). With
regard to the centrosome they interact with dynein, which in
turn helps to keep the centrosome close to the nucleus through
its microtubule-binding and microtubule minus end-directed
motor activity. Sun proteins are also linked to the nuclear lami-
na and this linkage is of crucial importance for proper centro-
some/nucleus attachment (Schneider et al., 2011).
The linkage of Sun-proteins with nuclear lamina proteins has
so far been proven in metazoans (Haque et al., 2006) and plants
(Graumann, 2014) and, thus, it is very likely that this holds true
for amoebozoans as well. Consistent with that the Dictyosteli-
um lamin NE81 is also required for centrosome/nucleus attach-
ment and interference with NE81 function caused phenotypes
similar to Sun1 disruptions in this organism (Xiong et al., 2008;
Schulz et al., 2009; Krüger et al., 2012). This suggests that la-
mins were ancestrally involved in centrosome attachment to
the nucleus and have coevolved with Sun-proteins. In this light
the absence of Sun-proteins in discicristata were in agreement
with the substitution of lamins by NUP-1. Yet, coevolution is
not strictly necessary, since plants have Sun-proteins but NM-
CPs instead of lamins. A requirement of KASH-domain prote-
ins for centrosome/nucleus attachment is also not universal,
since bona fide KASH domain proteins appear to be absent in
Dictyostelidae. Yet, this condition may be caused by secondary
loss, since the classical SUN/KASH-based LINC complexes are
found in almost all branches of the tree of life except some
amoebozoans and discicristates (Field et al., 2012). Especially
the ubiquitous presence of Sun-proteins and their conserved
capacity to bind directly to chromatin tempted Cavalier-Smith
to propose that this binding activity was the key step in the
original attachment of proto-ER membranes to heterochroma-
tin during the formation of the nucleus in the first eukaryotic
common ancestor (FECA) (Cavalier-Smith, 2010).
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