Post-transcriptional Regulation of Gene Expression, Ribonticleoprotein Complexes, 
and Nuclear Structures 
hnRNP proteins on postmitotic cells, however, 
revealed that at the end of mitosis the hnRNP 
complexes dissociate in the cytoplasm and the 
different proteins are transported to the nucleus 
separately. Some, including CI, C2, and U, like 
snRNP proteins and lamins, are transported imme- 
diately (early group) , v^^hile others, including Al , 
A2, Bl, B2, E, G, H, and L, are transported later 
(late group). Thus immediately following reas- 
sembly of the nuclear envelope at the end of mi- 
tosis, pairs of cells are detected in which some 
hnRNP proteins are in the nucleus and others in 
the cytoplasm. These observations show that 
hnRNP complexes are dynamic structures in that 
hnRNP proteins can dissociate from the com- 
plexes and return to the nucleus separately. 
It is possible that the association and dissocia- 
tion of the complexes are controlled by post- 
translational modifications. An interesting candi- 
date effector of such modifications is a cell 
cycle-regulated protein kinase that phosphory- 
lates the hnRNP C proteins. The phosphorylation 
results in the appearance of novel, electrophoreti- 
cally distinct forms of the proteins. These slower- 
migrating forms, which we term Cs proteins, are 
readily detectable by immunoblotting with anti- 
C protein monoclonal antibodies. 
We partially purified the kinase (termed Cs ki- 
nase) that phosphorylates the C proteins to form 
the Cs proteins (several hundredfold) by high- 
performance liquid chromatography (HPLC). 
The activity of this novel cell cycle-regulated 
protein kinase is increased during mitosis. It is 
distinct from the histone HI kinase component of 
maturation-promoting factor (MPF/c<^c2) . In ad- 
dition to the Cs kinase, two other protein kinases 
were found to phosphorylate the C proteins. One 
of these is the previously identified casein kinase 
type II. The phosphorylated residues are serines, 
but the specific sites on the C proteins that are 
phosphorylated by the three kinases have not yet 
been determined. 
Cs kinase activity was also detected in extracts 
of the yeast Schizosaccheromyces pombe. This 
should help in determining the relationship be- 
tween Cs kinase and MPY/cdc2 and in under- 
standing the function of the Cs kinase. It may also 
make it possible to identify the C proteins of S. 
pombe. Among the likely possible functions of 
the phosphorylations of the C proteins are a role 
in the assembly/disassembly of hnRNP com- 
plexes, an effect on the RNA-binding activity of 
these proteins, and a role in the transport of the C 
proteins to the nucleus. 
Immunofluorescence microscopy with mono- 
clonal antibodies to hnRNP proteins and photo- 
chemical crosslinking of RNAto proteins in living 
cells by ultraviolet light indicated that the A, B, C, 
L, M, and U proteins are nucleoplasmic — that is, 
they are nonchromatin proteins that are confined 
to the nucleus and excluded from nucleoli. The 
mRNP proteins that have been characterized so 
far are different from the known hnRNP proteins 
and are confined to the cytoplasm. Therefore the 
mRNAs must exchange most, if not all, of the pro- 
teins with which they are associated in the nu- 
cleus as they are transported to the cytoplasm. 
The dissociation of these proteins from the nu- 
clear mRNA and the subsequent binding of mRNP 
proteins must be an important aspect of nucleo- 
cytoplasmic transport. Understanding the mecha- 
nism that regulates the disassembly of hnRNP 
complexes prior to the transport of their proteins 
to the nucleus following mitosis may also be im- 
portant for understanding this nucleocytoplas- 
mic transport process. The most abundant pro- 
tein that is bound to mRNA in the cytoplasm is the 
poly (A) -binding protein. We have isolated cDNA 
clones for it from several organisms, determined 
its sequence, and produced antibodies to it. We 
are currently studying its function. 
122 
