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Nucleolar dense granules in cytochalasin-treated conidia of Neurospora crassa
TURIAN, Gilbert, BARJA, François, CAESAR-TON THAT, T.C.
TURIAN, Gilbert, BARJA, François, CAESAR-TON THAT, T.C. Nucleolar dense granules in cytochalasin-treated conidia of Neurospora crassa . Cell Biology International Reports , 1992, vol. 16, no. 12, p. 1265-1266
DOI : 10.1016/S0309-1651(06)80043-3
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Cell Biology lnternational Reports, Vol. 16, No. 12, 1992 1265 N U C L E O L A R DENSE GRANULES IN C Y T O C H A L A S I N - T R E A T E D
CONIDIA O F N E U R O S P O R A C R A S S A G. Turian, F. Barja and T.C. Caesar-Ton That*
Laboratory of General Microbiology, University of Geneva, Sciences III, 1211 Gen~ve 4, Switzerland
I N T R O D U C T I O N
We have recently found that differentiation in Allomyces of the female gametangia rich in both actin and ribosomes can be fully prevented by the anti-actin drug cytochalasin D, while the males, poor in both components, were insensitive to cytochalasin (Nguyen Thi and Turian, 1992). This has led us to hypothesize a role for F-actin in the still elusive mechanism (Darnell et al., 1990) of extxanuclear driving of the preribosomal components. Support for this hypothesis was recently obtained in conidia of Neurospora crassa treated with cytochalasin B and thus unable to polarly focus F-actin for germ tube outgrowth (Barja and Turian, 1992).
Nucleolar dense granules were observed very similar to those previously detected and analyzed in heat shocked conidia. Such granules are RNAproteins (Ton-That et al., 1981) which migrate into the cytoplasm from the nucleus during conidial recovery of their monopolar germination ability, after shift-down to 25°C (Ton-That and Turian, 1984).
M A T E R I A L S AND M E T H O D S
Conidia of N. crassa were produced on the standard Vogel's minimum medium at 30°C. Cytochalasin B (CB) was dissolved in DMSO diluted to a final concentra- tion of 0.1% DMSO. Pregerminated conidia were incubated for 2 h in the presence of 60 or 120 Ixg/ml CB, half of which being [3H]CB (Amersham). They were then fixed in 2% glutaraldehyde-osmic acid, thin sectioned and autoradiography made using Ilford L4 nuclear emulsion (Ton-That and Turian, 1984) and sections observed in a Zeiss M10 electron microsocope.
RESULTS AND DISCUSSION
Ultrathin sections passing through nuclei of CB-treated conidia display intra- and perinucleolar dense granules (Fig. 1). Their aspect and positioning suggest that they are of the same, RNAprotein-rich type of granules cytochemically character- ized in heat shocked conidia (Ton-That and Turian, 1984). As for the radioactive grains of [3H]CB visible in the midst of nucleoli, they should reveal the presence of intranuclear actin as target of the known depolymerization effect of the drug.
These results can be interpreted as a first indication in favour of our hypothesis that actin is in some way involved in the mechanism of monopolar transport of preribosomal RNA from the nucleoli to the cytoplasm. They further suggest that our previously detected nucleolar dense granules of RNAproteins, presumably preribo- somal" RNAs by analogy with those described in other organisms (see Nover et al., 1989), might have resulted from heat-induced actin depolymerization and/or its cross-linking. Such effects have been reported in mouse cells to be mediated by the 90 kDa heat shock protein (Koyasu et al., 1986) which was also detected in heat- treated conidia ofN. crassa by van Tuinen and Turian (1982).
*Present address: Montana State University, Bozeman, MT 59717-0056, USA.
0309-1651/92/121265-02/$08.00/0 © 1992 Academic Press Ltd
1266 Cell Biology lnternational Reports, VoL 16, No. 12, 1992
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Fig. 1. Intranucleolar dense granules visualized on autoradiography of thin sec- tioned conidia of N. crassa treated for 2 h with: (a) 60 p.g/ml cytochalasin B (half 3H-) showing dense, intra- and perinucleolar granules (arrow) and radioactive grains in the nucleolus center of a wide germ tube; (b) 120 p.g/ml CB (half 3H-) showing nucleolar dense granules (arrow) and central Ag grains. Bar = 1 p.m.
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