A GTPase for nuclear, import MELCHIOR, F., PASCHAL, B., EVANS, J. and GERACE, L. (1993) Inhibition of nuclear protein import by nonhydrolysable

analogues of GTP and identification of the small GTPase Ran/TC4 as an essential transport factor I. Cell Biol. 123, 1649-1659

MOORr:, M. S. and BLOBEL, G. (1993) The GTP-binding protein Ran/TC4 is required for protein import into the nucleus Nature 365, 661-663

Transport of most proteins and nu- cleic acids across the nuclear envelope is a specific and active process that occurs through nuclear pore com- plexes. In the past few years, per- meabilized cell systems have been developed that reconstitute nuclear protein import and allow character- ization of the cytosolic factors in- volved. These two papers report that one such factor is the Ras-related GTPase Ran/TC4.

Melchior et al. initially found that nonhydrolysable analogues of GTP, such as GTPyS, can inhibit nuclear import. This inhibition was accen- tuated by addition of a 20-30 kDa cytosolic fraction (about the size of many Ras-related GTPases) that was itself required for efficient transport.

Bacterially expressed Ran/TC4 pro- rein could mimic both the transport activity and the GTP~S inhibition observed with this fraction. Moore and Blobel found that Ran[i'C4 is a major component of one of two cytosolic fractions they previously showed to be sufficient to support nuclear import, in their assays, ad- dition of Ran/TC4 could stimulate transport under certain nonsatur- ating conditions and this stimulation was inhibited by GTP analogues. Thus, these papers show that GTP hydrolysis is required for nuclear import and one of the major GTPases involved is Ran/TC4.

Ran/TC4 was originally identified in mammalian cells and has also been studied in yeast. It is an abundant

protein that appears to be mainly located in the nuc!eus, although it might travel between the nucleus and cytoplasm. It interacts with another nuclear protein, RCC1, which acts as its guanine-nucleotide- exchange factor. Genetic studies in yeast have implicated RCC1 in a number of nuclear functions, in- cluding RNA transport, DNA repli- cation and cell cycle control.

It is not yet clear how Ran/TC4 is involved in nuclear import, and in particular whether it has a direct role in the transport process or acts as a regulatory factor. Further characterization of the system and the part played by GTPases will hopefully shed light on these points.

Thylakoid protein transport pathways

CLINE, K., HENRY, R., LI, C. and YUAN, J. (1993) Multiple pathways for protein

transport into or across the thylakoid membrane

EMBOI. 12, 4105-4114

Although the number of nucleus- encoded proteins transported to the thyla!~oid lumen within the chloro- plast is rather small, the manner in which these few proteins are tar- geted to this compartment is remarkably complex. The energy requirements for the transport of proteins to the lumen depends on which protein is being considered, with some using only ATP, others using only a protonmotive force and still others using a combination of both ATP and a protonmotive force. This raises the question of whether the distinct energy requirements reflect different protein translocation machineries, or whether the same transport apparatus operates undP.r different energy-coupled modes.

The paper by Cline et al. demon- strates that the different energetics are, in part, a manifestation of sep- arate transport pathways. Using pre- cursors produced in Escherichia coli, they .~howed that lumen- and membrane-resident thylakoid pro- teins can be classified according to whether they compete with one another in a thylakold transport assay; those within a set compete, but those in different sets do not. Interestingly, these same proteins did not sort into competition groups during transport across the chloro- plast envelope membranes.

The results reported in this paper suggest that the examined proteins utilize a common envelope transport apparatus, but disparate thylakoid translocation machineries. Interest- ingly, the competition groups for the thylakoid transport components are not precisely the same as those predicted on the basis of energetics. This competition assay will undoubtedly become a valuable tool in efforts to define the trans. location and integration pathways of both soluble and integral thylakoid proteins yet to be ex- amined.

HEADLINES The HEADLINES section of Trends in Cell Biology is intended to draw

attention to a selection of research papers of importance to cell biology that have been published in the past few months. Headlines are contributed regularly by a panel of research scientists appointed

by the Editor.

Please contact the TCB editorial team if you are interested in writing for the Headlines section.

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