Scientists
at the European Molecular Biology Laboratory (EMBL) in Heidelberg,
Germany, have combined the power of two kinds of microscope to produce a
3-dimensional movie of how cells ‘swallow’ nutrients and other
molecules by engulfing them. The study, published today in Cell,
is the first to follow changes in the shape of the cell’s membrane and
track proteins thought to influence those changes. It also provides
ample data to investigate this essential process further.

This
‘swallowing’, called endocytosis, is involved in a variety of crucial
tasks. It is used by brain cells relaying information to each other, for
instance, and is also hijacked by many viruses, which use it to invade
their host’s cells. When a cell is about to swallow some molecules, a
dent appears in the cell’s membrane, and gradually expands inwards,
pinching off to form a little pouch, or vesicle, that transports
molecules into the cell.

To
investigate how the cell’s machinery pulls in the membrane and forms
the vesicle, researchers led by Marko Kaksonen and John Briggs employed a
method they developed two years ago to faithfully follow the exact same
molecules first under a light microscope and then with the higher
resolution of an electron microscope. This enabled them to combine two
sets of data that so far could only be obtained in isolation: the timing
and sequence with which different components of the cell’s machinery
arrive at the vesicle-to-be, and the 3D changes to membrane shape that
ultimately form that vesicle. They discovered, for instance, that the
first proteins to arrive on the inside of the cell’s membrane are not
able to start bending it inwards until a network of the cell’s
scaffolding protein, actin, forms and starts pulling on the membrane.

The
data used to make the video is freely available to the scientific
community and will, Kaksonen and Briggs believe, provide valuable
information to others trying to develop physical models of how this
process works. The EMBL scientists themselves are probing the roles of
individual proteins in this process, by perturbing them, and would like
to extend the current work in yeast to human cells.

European Molecular Biology Laboratory