From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Mon Sep 06 1999 - 13:02:16 MDT
On Mon, 6 Sep 1999 hal@finney.org wrote:
>
> One note, from
> http://www.portlandpress.co.uk/books/isbn/1855781212.htm, is that 95%
> of the cell's lipid membranes are found internally, separating cellular
> compartments (mitochondria and other organelles, endoplasmic reticulum,
> the nucleus, etc.).
Quite possibly.
> So whatever method is used to pass through lipid membranes would be
> needed even more within the cell than for getting inside.
The cell goes to fairly complex lengths to not have to do this.
The proteins that are targeted to the endoplasmic reticulum
and everything downstream from that (golgi apparatus,
peroxisomes (probably), outer cell membrane), are actually
synthesized *into* the ER. So you don't have this
synthesize, fold, unfold, transport, refold, deliver process.
Instead it is synthesize while transporting, fold, deliver.
In contrast, I think all proteins that end up in the nucleus
are synthesized, folded, then delivered. Their import
into the nucleus is mediated by the nuclear pore complex
(which is probably a real example of a "gatekeeper").
I've never read anything that indicates proteins are
unfolded to get through the complex and I'm under the
impression that really large proteins can't get into
the nucleus. This mechanism is what makes me think
there is a limit on the size of a viral DNA genome that
can get transported into the nucleus. In theory you
could feed the DNA through the pore as a thread but
this seems like a *very* complex operation.
The interesting part is that all of these proteins have
"targeting sequences" for the nucleus, peroxisomes, ER,
mitochondria, etc. to get them to go to the right
place with a minimum requirement for refolding.
Transport is a *big* part of what higher cells have to manage.
Another interesting part of the whole cell structure is --
how does the nuclear pore mediate the transport of
newly synthesized proteins or activated transcription
factors *into* the nucleus and "old" grungy proteins or
newly synthesized mRNA *out-of* the nucleus. What
functions as the transport stop-lights?
Magic biology, (oh, nooooooo)...
Robert
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