plant surface microbiology.pdf

18 Mycorrhizal Development and Cytoskeleton 311
coiled-coil tail domains. The protein forms homotetramers and it is required
for nuclear division. A motor protein has been biochemically characterized
from Neurospora crassa (Steinberg and Schliwa 1996) and the zygomycete
Syncephalastrum racemosum (Steinberg 1997), and the production of an antibody
against the N. crassa protein helped to isolate the first fungal conventional
kinesin encoding gene, Nkin (Steinberg and Schliwa 1995). Using
probes derived from conserved regions of kinesins and screening existing
genomic databases, has led to the isolation of conventional kinesins from
Ustilago maydis (Lehmler et al. 1997), S. racemosum (Grummt et al. 1998),
Nectria haematococca (Wu et al. 1998), and A. nidulans (Requena et al. 2001).
Conventional kinesins belong to the fastest kinesins known in eukaryotic
cells, moving as dimers on MTs with a velocity of 2.5 µm/s in in vitro gliding
experiments, which is three- to fivefold faster than that of their animal counterparts
(Steinberg 1997; Grummt et al. 1998). The structural features responsible
for the high gliding velocity of fungal kinesins are not yet well understood,
but under active investigation (Kallipolitou et al. 2001).
The deletion of the conventional kinesin-encoding gene from any of the filamentous
fungi investigated leads to a reduction of polarized growth and the
size of Spitzenkörper, the secretory vesicle aggregation at the hyphal tip
(Lehmler et al. 1997; Seiler et al. 1997; Wu et al. 1998; Seiler et al. 1999; Requena
et al. 2001). These observations support the idea that conventional kinesin is
involved in the transportation of components necessary for hyphal growth
along the MTs towards the hyphal tip. It is noteworthy that in no case the
mutation of the conventional kinesin gene has led to a complete cessation of
growth, which implies the existence of other hyphal transportation systems,
either based on other less efficient kinesins, or on the actin – myosin system.
In A. nidulans, the deletion of kinesin also caused disturbance in nuclear distribution
in the germ tube and hyphae suggesting that conventional kinesin
plays a role in nuclear migration (Requena et al. 2001). In addition, the functional
analysis suggested that the conventional kinesin of A. nidulans is
involved in destabilization of MTs (Requena et al. 2001).
The MT-associated motor cytoplasmic dynein is generally proposed to
provide the motive force for nuclear movement in filamentous fungi (Morris
et al. 1995; Fischer 1999; Suelmann and Fischer 2000). The heavy chain of
cytoplasmic dynein is a large polypeptide with a region for dimerization at
the N-terminal and globular motor domain with four ATP binding and MTbinding
sites at the C-terminal portion of the polypeptide. In association with
the N-terminus of the dynein heavy chain, smaller polypeptides called intermediate
and light chains occur, which are involved in the regulation of dynein
heavy chain activity and function (Steinberg 1998, 2000). A multi-subunit
complex dynactin is also required for efficient MT-associated transport by
cytoplasmic dynein. In the dynactin complex actin-related protein ARP1 is
the most abundant and p150 Glued the largest subunit. The dynactin complex
also includes several other polypeptides that play an important role both in