Open Mitosis: Nuclear Envelope Dynamics 217
1993). Attempts to identify plant lamin-like antigens implicate coiled-coil
proteins as likely candidates, suggesting that this structural feature shared by
lamins is responsible for the cross-reaction with anti-lamin antibodies (Blu-
menthal et al. 2004). The precise targets of these antisera in plant cells are
unknown.
Several other antisera directed against mammalian proteins have been
used successfully in labeling the plant nuclear envelope. One example is
a monoclonal antibody against calf thymus centrosomes, which might cross-
react with microtubule organizing components at the plant nuclear envelope
(Schmit et al. 1994). Another example is a peptide antibody raised against
LCA1, a SERCA-type ATPase of mammalian ER. This antibody localizes to nu-
clear envelopes in tomato cells with high specificity both at light microscopic
as well as electron microscopic levels and was found to relocalize to spindle
poles during mitosis (Downie et al. 1998). An antiserum against chicken spec-
trin also decorated the plant nuclear envelope in pea (de Ruijter et al. 2000).
Spectrins are nuclear envelope proteins specific to animals with no homologs
in plants and are involved in the attachment of actin filaments at the nuclear
membrane.
While immunological studies provide snapshots of nuclear envelope be-
havior in fixed cells, tagged proteins can provide dynamic data of mitotic
processes in living cells. A heterologous nuclear envelope protein suitable for
use as such a marker in plant cells is the mammalian lamin B receptor (LBR).
This protein is typically only found in animal cells, but not in plants. LBR
is an integral membrane protein of the inner nuclear membrane and binds
to chromatin via its N-terminus (Pyrasopoulou et al. 1996; Ye and Worman
1994). An adjacent region and the first transmembrane domain are respon-
sible and sufficient for the nuclear envelope targeting of the protein (Smith
and Blobel 1993; Soullam and Worman 1993, 1995). While mammalian LBR
is tethered inside the nucleus via interaction with lamin B, it appears that
its chromatin-binding capabilities are sufficient to ensure similar tethering at
the inner nuclear membrane in plant cells that do not possess lamins. This
phenomenon has been utilized to visualize the plant nuclear envelope during
mitosis in live cells by using the N-terminal 238 amino acids of LBR fused to
green fluorescent protein (Irons et al. 2003).
Several plant nuclear envelope proteins have been identified and utilized
for plant nuclear envelope dynamics studies (see Table 1). These include
NMCP1 from carrot (Masuda et al. 1999), the tomato protein MAF1 (Dixit and
Cyr 2002), and the Ran cycle component RanGAP (Jeong et al. 2005).
NMCP1 was the first nuclear envelope protein cloned from plant cells. It
is a 134-kDa protein found exclusively at the nuclear periphery during inter-
phase in immunostaining and relocates to the spindle poles during mitosis
(Masuda et al. 1997, 1999). NMCP1 staining of the nuclear envelope disperses
during late prophase, metaphase, and anaphase with a concentration in the
spindle region. In late anaphase, staining becomes visible around the polar re-