quired for post-translational protein translocation 
in yeast. This unexpected result has made a major 
impact on assigning functions to these proteins. In 
addition, Drs. Chirico and Water have shown that 
another A^-ethylmaleimide-sensitive factor is re- 
quired. It is likely that this factor represents a sig- 
nal-recognition factor. 
Dr. Jacques YaDeau has succeeded in solubilizing 
yeast microsomal signal peptidase and developed 
an in vitro assay for measuring its activity. This 
work should pave the way for the isolation of the 
enzyme. 
The substrate that was used in Dr. YaDeau's work 
was yeast prepro-a factor. Drs. Gerald Waters and 
Emily Evans have shown that this protein has a 
cleavable signal sequence. 
II. Prokaryotic Plasma Membrane Translocon. 
Previously this laboratory showed that a soluble 
factor (presumed to be a signal-recognition factor) 
was required for protein translocation across the E. 
coli plasma membrane. Dr. Makoto Watanabe has 
now purified this factor to homogeneity. This pro- 
tein is a 64 kDa homotetramer consisting of four 
identical 16 kDa subunits. Amino-terminal se- 
quence analysis revealed that the 16 kDa protein is 
identical to the secB gene product. Dr. Watanabe 
has shown that the secB tetramer functions as a sig- 
nal-recognition factor that binds to the signal se- 
quence of preproteins and can compete with the 
signal-recognition particle of the ER translocon for 
binding to the signal sequence. 
Dr. Watanabe also has obtained biochemical evi- 
dence that PrlA (SecY) of E. coli functions as a 
membrane-bound signal-recognition system. Fab 
fragments of antibodies raised against synthetic 
peptides representing the amino- and carboxyl-ter- 
minal domains of this protein inhibited transloca- 
tion by interfering with the binding of the pre- 
proteins to the membrane. 
III. Chloroplast Envelope Translocon. 
Using an antiidiotypic antibody approach, Drs. 
Debkumar Pain and Yashpal Kanwar have identified 
the first component of the chloroplast envelope 
translocon. Antibodies were raised in rabbits 
against a synthetic signal peptide. An IgG fraction 
of the anti-signal peptide antibodies was then used 
to raise antiidiotypic antibodies that mimic the sig- 
nal peptide. Fab fragments of these antibodies in- 
hibited protein import into chloroplasts. Polypep- 
tides of the chloroplast envelope resolved by SDS- 
PAGE, transferred to nitrocellulose, and then 
probed with the antiidiotypic antibodies reacted 
with a 30 kDa integral membrane protein. Immu- 
nofluorescence of isolated chloroplasts yielded 
punctate staining, and immunoelectron microscopy 
showed decoration of the so-called contact zones 
between outer and inner chloroplast membrane. It 
is likely that this protein is a signal sequence-bind- 
ing subunit of a protein-conducting channel in the 
outer chloroplast membrane that is linked to a sim- 
ilar channel in the inner chloroplast membrane, 
thus giving rise to the morphological phenotype of 
contact zones. 
IV Mitochondrial Membrane Translocon. 
Except for signal peptidase, none of the compo- 
nents of the mitochondrial membrane translocon 
have been isolated. Drs. Hiroshi Murakami and 
Debkumar Pain have demonstrated that protein im- 
port into mitochondria requires hsp70 (see ER 
translocon of yeast) and an A^-ethylmaleimide-sen- 
sitive cytosolic factor. The latter is presumed to be a 
signal-recognition factor. 
V. Structure and Function of the Nuclear Lamina 
and Pore Complexes. 
A. Higher eukaryotes. Drs. Howard Worman and 
Spyros Georgatos and graduate student Jeffrey 
Yuan have identified a 58 kDa lamin B receptor in 
the nuclear envelope of turkey erythrocytes. The 
protein is an integral membrane protein. 
Drs. Richard Wozniak and Eckart Bartnik have 
cloned and sequenced the cDNA for an integral 
membrane glycoprotein (gp210) that is localized in 
the membrane limiting the nuclear pore. The de- 
duced amino acid sequence revealed that gp210 
has one classical transmembrane segment and an- 
other hydrophobic region that could fijnction as a 
fusogenic peptide. It is possible that this protein 
fianctions in the formation of pores and in anchor- 
ing of the nuclear pore complex. 
B. Yeast. Dr. John Aris has identified two nuclear 
envelope proteins in yeast that cross-react with an- 
tisera against mammalian nuclear pore complex 
proteins. Immunofluorescence and electron mi- 
croscopy data suggest that these cross-reactive pro- 
teins are yeast nuclear pore complex proteins. Drs. 
Spyros Georgatos and lonna Maroulakou have 
used various binding assays and cross-reactive anti- 
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