Intracellular Protein Traffic and Nuclear Organelles 
distinct structures. One, the nuclear pore com- 
plex (NPC), is located in the nuclear envelope. 
We speculate that this organelle is involved in 
"gene gating"; i.e., each pore complex is at- 
tached to a set of actively transcribed genes. We 
have identified and isolated several proteins of 
the NPC of mammalian cells. So far we have es- 
tablished the primary structure of 3 of the more 
than 100 proteins of this large organelle. Re- 
cently we succeeded in isolating NPCs from 
yeast. These genetically amenable cells should fa- 
cilitate the structural and functional analysis of 
the many NPC proteins. We have also established 
the primary structure of two membrane proteins 
that are specifically located in the pore mem- 
brane domain of the nuclear envelope mem- 
brane. We are using cell-free systems for protein 
uptake into the nucleus to isolate the hypotheti- 
cal SRF, the homing receptor, and the so-called 
transporter of the pore complex. 
The other morphologically distinct structure 
associated with the nuclear envelope that we are 
studying is the nuclear lamina, a fibrous mesh- 
work associated with the nuclear side of the inner 
nuclear envelope membrane. The lamina consists 
of three proteins, which we have termed lamins 
A, B, and C. We have speculated that the nuclear 
lamina is involved in the three-dimensional orga- 
nization of nontranscribed chromatin. 
By molecular cloning and cDNA sequencing of 
the three lamins, we and others showed recently 
that these proteins are members of the interme- 
diate filament protein family. We demonstrated 
that lamin B binds to the carboxyl-terminal por- 
tion of cytoplasmic intermediate filament pro- 
teins and that ankyrin, a protein associated 
with the plasma membrane, binds to the amino- 
terminal portion of cytoplasmic intermediate fila- 
ment proteins. Recently we have investigated the 
interaction of such a protein with nuclear lamin B 
in more detail and have localized the interacting 
regions to the near carboxyl-terminal segment of 
these proteins. Interestingly, a synthetic peptide, 
representing this lamin B-binding site of the cy- 
toplasmic intermediate filament protein, when 
microinjected into cells, led to the detachment of 
intermediate filaments from the nucleus. 
These data indicate a direct connection be- 
tween the plasma membrane skeleton (ankyrin) , 
the cytoskeleton (intermediate filaments), and 
the peripheral nucleoskeleton (lamina) through 
the NPC. Moreover, we recently identified a la- 
min B receptor in the inner nuclear membrane. 
Its primary structure has been determined by mo- 
lecular cloning and cDNA sequencing. This re- 
ceptor has a number of interesting sites, suggest- 
ing that it may not only interact with lamin B but 
also with DNA. 
46 
