However, after evacuation of the translocating 
chains by ribosome-mediated coupling to puromy- 
cin, large channels, each of a conductance of 220 pS 
at 50 mM KCl, could be detected. These channels 
closed abruptly when the ribosome was removed at 
high KCl concentrations. 
In the past year, Drs. Simon and Blobel investi- 
gated how these large PCCs would be opened. 
Could the PCC be a ligand-gated channel, i.e., 
opened by the signal sequence of the translocating 
polypeptide as predicted in the signal hypothesis? 
To answer this question a signal peptide was chemi- 
cally synthesized and tested in a planar bilayer sys- 
tem, this time containing fused vesicles of the Esche- 
richia co// plasma membrane. Addition of the signal 
peptide in subnanomolar concentrations specifi- 
cally to the cytoplasmic side of the fused vesicle 
membrane revealed channels of similar conduc- 
tance, 220 pS at 50 mM KCl, as seen in mammalian 
rough microsomes. These data indicated that PCCs 
are indeed signal-sequence gated. Thus the princi- 
pal function of the various membrane-specific sig- 
nal sequences is to open cognate PCCs in intracellu- 
lar membranes. 
Transport on Intranuclear Tracks 
Another seminal discovery of the past year was the 
detection by Drs. Thomas Meier and Blobel of in- 
tranuclear tracks, up to several microns in length, 
on which transport between nucleoli and a small 
number of dedicated NPCs appears to occur. These 
tracks were detected through immunolocalization 
of Noppl40 (nucleolar phosphoprotein of 140 
kDa). Drs. Meier and Blobel showed that Noppl40 
shuttles between the nucleolus and the cytoplasm. 
Immunoelectron microscopy revealed that Nopp- 
140 was localized in striking curvilinear arrays that 
extended from the nucleolus to some of the NPCs. 
Could the^e tracks be actin filaments? And if so, 
would Sj-type myosin motors move cargo (such as 
Noppl40) on these tracks? Are all of the intranu- 
clear transcription and RNP assembly sites con- 
nected by tracks to NPCs? Are these tracks reversibly 
disassembled during mitosis? These are only a few 
questions elicited by these findings. The answers are 
bound to affect profoundly current thinking about 
nuclear architecture and processes such as tran- 
scription, DNA replication, and mitosis. 
Two Distinct Cytosolic Fractions 
for Protein Targeting and Translocation 
through the Nuclear Pore Complex 
Drs. Mary Moore and Blobel have identified two 
cytosolic fractions from Xenopus oocytes that con- 
tain the activity necessary to support both steps of 
nuclear import in digitonin-permeabilized mam- 
malian cells: binding at the nuclear envelope 
and translocation through the NPC. The first cyto- 
solic fraction (fraction A) interacts with an import- 
competent, but not a mutant, nuclear localization 
sequence-bearing conjugate and stimulates its 
accumulation at the nuclear envelope in an ATP- 
independent fashion. The second cytosolic fraction 
(fraction B) gives no discernible effect when added 
alone; but when added together with fraction A, or 
after fraction A, it stimulates the passage of the con- 
jugate from the outer nuclear envelope to the inte- 
rior of the nucleus in an ATP-dependent fashion. 
Dr. Blobel is also Professor of Cell Biology at the 
Rockefeller University. 
Articles 
Chaudhary, N., McMahon, C, and Blobel, G. 
1991. Primary structure of a human arginine-rich 
nuclear protein that colocalizes with spliceosome 
components. Proc Natl Acad Sci USA 88:8189- 
8193. 
Meier, U.T., and Blobel, G. 1992. Noppl40 shut- 
tles on tracks between nucleolus and cytoplasm. 
Ce// 70:127-138. 
Migliaccio, G., Nicchitta, C.V., and Blobel, G. 
1992. The signal sequence receptor, unlike the 
signal recognition particle receptor, is not essen- 
tial for protein translocation. / Cell Biol 117:15- 
25. 
Moore, M.S., and Blobel, G. 1992. The two steps of 
nuclear import, targeting to the nuclear envelope 
and translocation through the nuclear pore, re- 
quire different cytosolic factors. Cell 69:939- 
950. 
Simon, S.M., and Blobel, G. 1992. Signal peptides 
open protein-conducting channels in E. coli. Cell 
69:677-684. 
Soldatov, N.M. 1992. Molecular diversity of L-type 
Ca^^ channel transcripts in human fibroblasts. 
Proc Natl Acad Sci USA 89:4628-4632. 
28 
