Relation between microns and kD - (Feb/28/2011 )

What is the relation between microns and kD? I know there is a list with 0.1um/1000kD - 0.0015um/5kD. But a membrane for westernblot has a pore size of 0.45um... but proteins of 20kD are still there... is that only because the denaturation (no 3D foldings)?
What kind of factors (proteins?) can't pass a 0.2um filter in there folded form?
If I see the list of relation between micron and kD, 0.2micron must be very big and factors upto 2000-3000kD will pass... is that true or will I miss something in my reasoning?

No, your reasoning is perfectly sound... the micron scale is much bigger than most proteins. The reason they do not pass is that the membranes are charged and bind the protein.

Ok, thank you! But the other question is still there...
We are using a normal 0.2um filter and we are expacting that all proteins will pass the filter... but we are loosing some factors for our cells. What kind of factors (proteins?) can't pass a 0.2um filter? If by a 0.1um filter will pass proteins to 1000kD, what will a 0.2um filter get out of the culture medium?

Some proteins aggregate and these won't pass through filters easily. Other proteins are charged and may bind to the filter.