Intracellular Protein Traffic and 
Nuclear Organelles 
Giinter Blobel, M.D., Ph.D. — Investigator 
Dr. Blobel is also Professor of Cell Biology at the Rockefeller University. He received his M.D. degree from 
the University of Tiibingen and his Ph.D. degree with Van Potter in oncology from the McArdle Laboratory 
at the University of Wisconsin-Madison. Thereafter, he did postdoctoral work with George Palade at 
Rockefeller. Dr. Blobel is a member of the National Academy of Sciences and of several other distinguished 
societies. He has received many honors, including the Gairdner Foundation Award. 
NUMEROUS structurally and functionally di- 
verse proteins can be translocated across a 
few distinct cellular membranes. It is now estab- 
lished that targeting to these membranes and 
translocation across them is specified by a mem- 
brane-specific "signal" sequence. This is a part, 
transient (or permanent), of the protein to be 
translocated. The primary structure of numerous 
representatives for such a sequence has been es- 
tablished in our laboratory and others. Present 
work focuses on the identification and character- 
ization of machinery involved in the recognition 
of a signal sequence, in its targeting to the proper 
membrane, and in protein translocation. Collec- 
tively, this machinery comprises what has been 
termed a protein translocon. 
Our laboratory is working on translocons for 
four distinct cellular entities, namely 1) the en- 
doplasmic reticulum (ER) of animal (and yeast) 
cells, which is able to translocate proteins from 
the cytosol to the ER lumen; 2) bacterial plasma 
membranes (gram-negative bacteria), able to 
translocate proteins from the cytoplasm to the 
periplasmic space; 3) yeast mitochondria, able to 
translocate protein from the cytoplasm to the mi- 
tochondrial interior ("matrix") across outer and 
inner mitochondrial membranes; and 4) plant 
cell (pea and spinach) chloroplasts, able to trans- 
locate protein from the cytoplasm to the chloro- 
plast interior ("stroma") across outer and inner 
chloroplast membranes. 
From studies on these four translocons so far, 
but especially on the ER one, it is likely that these 
and others are composed of at least four entities: 
1) a soluble signal recognition factor (SRF), 2) 
a homing receptor, 3) a signal sequence-gated 
protein- conducting channel, and 4) a signal-re- 
moving peptidase. SRF has as its functions «) rec- 
ognition of the signal sequence and b) targeting 
to the homing receptor, which is restricted in its 
localization to a translocon-specific membrane. 
Interaction of the signal sequence-SRF complex 
with the homing receptor leads to a) dissociation 
of the signal sequence and b) its presentation to a 
signal sequence receptor, which might be a sub- 
unit of the protein-conducting channel. This 
channel would close immediately upon comple- 
tion of translocation, only to open again after pre- 
sentation of another signal sequence. The pepti- 
dase would, in most cases, remove the signal 
sequence either during translocation or shortly 
thereafter. 
In the case of the ER, the SRF was isolated and 
shown to be a ribonucleoprotein particle. Re- 
ferred to as a signal recognition particle (SRP), 
this consists of one 7S RNA molecule and six dif- 
ferent proteins. Likewise, a homing receptor (re- 
ferred to as SRP receptor) and a signal-removing 
peptidase, a complex of five proteins, were iso- 
lated. More recently we were able to demonstrate 
the existence of a protein-conducting channel, 
using electrophysiological methods. The protein 
constituents of this channel remain to be identi- 
fied. We have been able to solubilize the ER 
membranes by detergent and to reconstitute 
translocation-competent vesicles. Using this 
method, it should be possible to identify the 
channel proteins. 
We recently identified components of other 
translocons. An SRF was isolated for signal se- 
quence targeted to the bacterial plasma mem- 
brane. Moreover, we identified signal sequence- 
binding subunits for the mitochondrial and 
chloroplast translocons. These integral mem- 
brane proteins are located in contact sites be- 
tween outer and inner organelle membranes and 
are candidates for subunits of a protein-conduct- 
ing channel in the outer organelle membrane 
linked to a protein-conducting channel in the in- 
ner membrane. 
Our other major research effort focuses on the 
organelles associated with the cell's nuclear en- 
velope membranes. These organelles are thought 
to organize the large amount of information in 
the linear structure of the DNA into numerous 
structurally and functionally distinct three- 
dimensional superstructures, allowing only a lim- 
ited amount of that information to be expressed. 
Characterization of these organelles should ad- 
vance understanding of such fundamental pro- 
cesses as differential gene expression, cell differ- 
entiation, and development. 
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