MadSci Network: Cell Biology |
The
nucleus is surrounded by a membrane which serves to keep some molecules in (like
proteins that interact with DNA) and some moleucules out (proteins that are
required elsewhere in the cell). Other molecules have to be both inside the
nucleus and outside the nucleus. Messenger RNA (mRNA) for instance, gets made
inside the nucleus and then leaves the nucleus for use by ribosomes. In order
to let certain molecules out and to keep other molecules in, the nucleus is
studded with holes (called pores) which are filled with a large protein complex
called the nuclear pore complex (NPC). NPCs are therefore the sole site of flow
into and out of the nucleus. Being such a large structure, the NPCs create
plugs or barriers. But, given their structure (see image of a single NPC-in
color, embedded in the membrane of the nucleus-the "U" shapes at either side of
the NPC), some small molecules can sneak through. These would be ions (like
calcium) and small proteins (less than 40 kiloDaltons). Larger molecules must
make their way through the central channel of the NPC. To do so, these larger
molecules must contain within their sequence particular stretches of amino acids
(these are called NLSs for nuclear localization sequences or NESs for nuclear
export sequences). NLSs and NESs are recognized by import or export receptors
(proteins which recognize the amino acid sequences of other proteins), depending
on which way (in or out) the transport is happening. It's not totally clear how
these receptors allow large molecules to go through the central NPC channel but
it probably has to do with causing a change in the NPC which makes the channel
bigger, say by removal of the central plug (shown in red). There are also other
proteins (called transport factors) which use energy (ATP/GTP) to help the
receptors to dock proteins and RNAs at the NPC and then to translocate them
through.
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