Membrane Lipids and Cell Regulation 
John A. Glomset, M.D. — Investigator 
Dr. Glomset is also Professor of Medicine and Biochemistry at the University of Washington School of 
Medicine. He received his M.D. and M.D. /Ph. D degrees from the University of Uppsala, Sweden. He then 
joined the Department of Medicine at the University of Washington. He received an honorary M.D. degree 
from the University of Oslo for his discovery of a plasma enzyme, lecithin :cholesterol acyltransferase 
(LCAT). Dr. Glomset is a member of the National Academy of Sciences. 
SEVERAL years ago, while studying the replica- 
tion of cells in culture, we discovered that 
compactin, a drug that can prevent the biosynthe- 
sis of a key metabolic intermediate called meva- 
lonic acid, can also block the synthesis of DNA 
and prevent cells from attaching normally to a 
culture dish. Furthermore, we showed that com- 
pactin 's effects on DNA synthesis and cell attach- 
ment depended on an induced deficiency of 
mevalonic acid. Both effects could be prevented 
by adding mevalonic acid to the cell culture me- 
dium, though known products of mevalonic acid 
metabolism, including cholesterol, were inac- 
tive. Thus it appeared that either mevalonic acid 
itself or an unknown product was involved. 
In an attempt to identify the compound that 
was required for DNA synthesis and cell attach- 
ment, we treated cells with compactin in the pres- 
ence of mevalonic acid with a radioactive tracer. 
Just enough exogenous mevalonic acid was used 
to prevent the drug's effects. Then we analyzed 
the radioactive products of mevalonic acid that 
the cells formed. This led to the discovery of a 
new class of modified proteins in animal cells. 
As much as 40 percent of the cell-associated 
radioactivity was attached to a special group of 
cell proteins that could be distinguished by gel 
electrophoresis. The radioactivity seemed to be 
present in protein-attached products of meva- 
lonic acid, called isoprenoid compounds. The 
discovery of this new type of modified proteins 
opened up a field of protein research that is now 
expanding rapidly. 
In follow-up studies, we sought to identify the 
proteins and characterize the attached isoprenoid 
groups. We were able to show that one of the 
modified proteins is lamin B, a structural protein 
that is attached to the inner nuclear membrane. 
In collaboration with Michael Gelb (University of 
Washington), we demonstrated that lamin B is 
modified by a cysteine thioether-linked farnesyl 
group and that the farnesylated cysteine residue 
is located at the protein's carboxyl-terminal end. 
(A farnesyl group is a 1 5 -carbon isoprenoid com- 
pound that is formed from three molecules of 
mevalonic acid.) 
These results are of considerable interest, as 
other investigators have identified additional 
proteins, including the oncogene product p21 
ras, that are similarly modified. Moreover, there 
is reason to believe that the modified carboxyl- 
terminal cysteine residues in lamin B and p21 ras 
promote binding of the respective proteins to 
cell membranes. 
Some animal cell proteins are modified by far- 
nesyl groups; other animal cell proteins are modi- 
fied by another isoprenoid group, the 20-carbon 
geranylgeranyl moiety. In collaboration with Ber- 
nard Fung and Stephen Clarke (University of Cali- 
fornia, Los Angeles) and Masahito Kawata and 
Yoshimi Takai (Kobe University, Japan), we re- 
cently identified several geranylgeranylated pro- 
teins, including the 7-subunits of a brain hetero- 
trimeric guanine nucleotide-binding protein 
and three low-molecular-weight guanine nucleo- 
tide-binding proteins — smg p21B, smg p25A, 
and G25K. 
These results and those of other investigators 
have raised a number of important questions. At- 
tempts to identify and characterize the enzymes 
that catalyze the various modification reactions 
are in progress in several laboratories. Work in 
our laboratory, done in collaboration with Fuyu- 
hiko Tamanoi (the University of Chicago), has 
provided evidence that the farnesyl transferase ac- 
tivity of the yeast Saccharomyces cerevisiae de- 
pends on two different genes, DPR 1 /Rami and 
Ram2, whereas the geranylgeranyl transferase ac- 
tivity depends on a third gene, CDC43- The pre- 
cise role of these genes remains to be 
determined. 
Another important question relates to the func- 
tional significance of the modification reactions. 
Many of the modified proteins appear to be at- 
tached to membranes, and the modifying iso- 
prenoid groups appear to promote this attach- 
ment. But it is not clear how the modifying 
groups interact with the membranes at a molecu- 
lar level. Experiments that address this question 
are under way. 
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