MANY freshly made proteins are carried in tiny bags made from cell membrane
to the place in the cell where they are needed. This delivery system is critical
to the cell’s survival. But how do those packages find the right destination?
New findings suggest that they carry an address in the form of protruding
protein arms.
The fatty bags, called vesicles, shuttle proteins between the cell’s many
membrane-bound compartments. For example, many newly made proteins enter the
endoplasmic reticulum (ER). Vesicles that bud off the ER carry these proteins to
the Golgi apparatus, where they are modified. Others take proteins to vacuoles,
the cell’s trash cans, while yet others deliver proteins to the cell’s outer
membrane. When a vesicle reaches its destination, it fuses with the membrane and
disgorges its contents.
Driving this fusion are two proteins called v-SNARE and t-SNARE, which
protrude from the membranes of both the vesicle and target and lock arms in a
sort of secret handshake. James Rothman and his colleagues at the Memorial
Sloan-Kettering Cancer Center in New York and other researchers have reproduced
the fusion event in a test tube by putting v-SNARE on one artificial vesicle and
t-SNARE on another.
Fusion is one thing, but how does a vesicle “know” it’s at the right target?
Is this also the work of the SNAREs? Rothman knew that each compartment marks
the vesicles it produces with a specific v-SNARE protein and that different
targets have different t-SNAREs. So using the test tube fusion system, he and
his team tested the ability of different SNARE pairs to drive fusion.
The SNAREs turned out to…