SELECTIVE IMPORT, 
THE JOB OF RECEPTOR 
PROTEINS 
Animal cells are surrounded by blood 
that contains many vital substances, 
some of which are present only in 
minute quantities. From this rich 
blend of ingredients, each cell must 
extract only those things it needs, 
and reject anything it is not equipped 
to handle. Such selective import is 
performed by specialized surface 
membrane proteins that are collec- 
tively called receptors. The unique 
characteristics of a cell depend, in 
large measure, on what kinds of 
receptors it has. Like a lock that 
accepts only an appropriately 
shaped key, each different receptor 
will function only when the correctly 
shaped blood-borne molecule (called 
a ligand) attaches to it. 
One important receptor is the LDL 
(low-density lipoprotein) receptor, 
which admits cholesterol into the cell. 
When a normal cell needs more cho- 
lesterol than it has produced, it syn- 
thesizes more LDL receptors. The 
liver manufactures cholesterol and re- 
leases it into the blood, and it is 
carried by LDL's to the waiting LDL re- 
ceptors. When the LDL's enter the 
cell, lysosomes break them down 
and release the cholesterol. As soon 
as the cholesterol in the cytoplasm 
reaches a certain concentration, the 
cell stops making new LDL receptors. 
LDL receptors bound to LDL's do 
not enter the cell one at a time. 
Rather, the complexes are rounded 
up and migrate to a place on the 
membrane where the surface begins 
to sink inward. These places, called 
coated pits, eventually pinch off and 
move into the cell. During this 
process, bits of the surface membrane 
are brought into the cell along with 
the LDL complexes. However, the cell 
does not become smaller, because 
the "lost" membrane is replaced by 
vesicles moving outward from the 
cell's interior. It has been calculated 
that an area of membrane the size of 
the entire cell surface is recycled in 
this way every 50 minutes. 
Not all of the molecules that influ- 
ence cell function are absorbed by 
the cell. Many hormones, for in- 
stance, exert their effects in an 
indirect manner, through receptor 
proteins that transduce the signal and 
generate "second messengers" within 
the cell. One of the best understood 
of these second messenger systems 
employs proteins called G proteins 
because they add phosphate atoms 
to a substance that contains guanine. 
In the G protein system, when a 
"first messenger" (such as a hormone) 
reaches the cell surface, it binds to a 
receptor that then sends a signal to a 
G protein located on the cytoplasmic 
side of the cell membrane. Depend- 
ing on its type, the activated G 
protein then either stimulates or inhibits 
the activity of any of a number of 
enzymes, including one called adeny- 
late cyclase. This enzyme causes 
cyclic AMP, a common second 
messenger, to be produced. Cyclic 
AMP then sets off a chain reaction 
that eventually results in changes in 
the shapes of certain proteins in the 
cell, which, in turn, lead to still other 
cellular responses. One of these 
