ylation. It is, however, distinct from the MARCKS 
protein in that it is encoded by a completely differ- 
ent gene and shows differences in developmental 
and tissue-specific expression. 
The cellular functions of both the MRP and 
MARCKS proteins are unknown, although attributes 
include calmodulin binding and probably actin 
binding. Studies under way to elucidate functions 
for the proteins include microinjection of antibod- 
ies to the proteins in intact cells; the use of antisense 
DNA to inhibit the biosynthesis of these proteins; 
and, ultimately, the use of homologous recombina- 
tion to disrupt the endogenous genes encoding 
these proteins and the evaluation of the resulting 
phenotypes. A number of chimeric mice have al- 
ready been created for the MARCKS gene, and their 
offspring are being evaluated for targeted disruption 
of one endogenous allele. At the same time, disrup- 
tion constructions are being prepared for the MRP 
gene, which was cloned and sequenced by Dr. David 
Lobach in the laboratory earlier this year. The goal 
of these studies will be the formation of mice, and 
cell lines derived from them, completely defective 
in the MARCKS and MRP genes; these animals and 
cells should be useful in determining the function 
of these proteins in normal physiology. 
Dr. Blackshear is also Professor of Medicine and 
Professor of Biochemistry at Duke University Med- 
ical Center. 
Articles 
Blackshear, P.J. 1992. Early protein kinase and bio- 
synthetic responses to insulin. Biochem Soc 
Trans 20:682-685. 
Blackshear, P.J., Tuttle, J.S., Oakey, R.J., Seldin, 
M.W., Chery, M., Phillip, C, and Stumpo, D.J. 
1992. Chromosomal mapping of the human 
{MACS) and mouse {Macs) genes encoding the 
MARCKS protein. Genomics 14:168-174. 
Blackshear, P.J., Verghese, G.M., Johnson, J.D., 
Haupt, D.M., and Stumpo, D.J. 1992. Character- 
istics of the F52 protein, a MARCKS homologue./ 
Biol Chem 267:13540-13546. 
Lee, R.M., Cobb, M.H., and Blackshear, P.J. 1992. 
Evidence that extracellular signal-regulated ki- 
nases (ERKs) are the insulin-activated Raf-1 ki- 
nase kinases. /B/o/ Chem 267:1088-1092. 
Manzella, J.M., and Blackshear, P.J. 1992. Specific 
protein binding to a conserved region of the or- 
nithine decarboxylase mRNA 5'-untranslated re- 
gion, f Biol Chem 267:7077-7082. 
PROTEIN TRAFFIC ACROSS INTRACELLULAR MEMBRANES 
GuNTER Blobel, M.D., Ph.D., Investigator 
Most cellular membranes contain systems for the 
unidirectional translocation of proteins across these 
membranes. The nuclear envelope is unique among 
cellular membranes in that it contains an organelle, 
the nuclear pore complex (NPC), that allows bidi- 
rectional transport, in and out of the nucleus, not 
only of proteins but also of RNPs and most likely 
also of DNPs. Most of the research efforts of Dr. Blo- 
bel and his colleagues are aimed at a detailed molec- 
ular characterization of these transport processes. 
For unidirectional translocation, protein-con- 
ducting channels (PCCs) are involved. These PCCs 
can open to an estimated diameter of 2-3 nm. There- 
fore proteins can be translocated only in an un- 
folded configuration. In addition to their ability to 
open and close across the membrane, PCCs can also 
open and close in a second dimension, toward the 
bilayer, and therefore effect integration of proteins 
into the lipid bilayer. 
NPCs are huge transporters (25 times the mass of 
a ribosome) that can open to a diameter of ~ 25 nm. 
Transport therefore does not require unfolding. Un- 
like PCCs, NPCs cannot integrate proteins into the 
bilayer. 
Protein-conducting Channel is Gated Open 
by Signal Sequence 
In 1990, Drs. Sanford Simon and Blobel used elec- 
trophysiological methods to show the existence of a 
PCC in the mammalian endoplasmic reticulum. 
Mammalian rough microsomal vesicles were fused 
to a planar bilayer. Each rough microsomal vesicle 
contains dozens of attached ribosomes in the pro- 
cess of translocating a polypeptide chain across the 
membrane. Therefore each microsomal vesicle con- 
tains dozens of PCCs. These PCCs were found to be 
electrophysiologically silent, i.e., impermeable to 
ion fluxes when occupied by translocating chains. 
CELL BIOLOGY AND REGULATION 27 
