Re: Questions about lectins and protein separation

Hi Robbie,

Thanks for your question - it's great to get a "classic biochemistry" query once in a while.

First, a little background on lectins. The name refers to a large group of proteins with a common property - they bind to glycoproteins. What's a glycoprotein? It's a protein that has oligosaccharides (sugars) covalently attached to it. This is an example of a post-translational modification and is very common in eukaryotic organisms - up to 50% of proteins may be glycosylated.

Back to lectins. Lectins were found first in plants and plant lectins are the ones that we use most in biochemistry, but they are also found in animal cells e.g. in the mammalian liver. Their biological roles are diverse and in many cases, not known for sure. However, they have found application in protein purification because different lectins bind to different oligosaccharides with different specificities.

Let's look at the three lectins that you mentioned. Concanavalin A (ConA for short) from the jackbean is widely used. It binds to alpha-mannosyl groups which are a core component of many sugars, so it could be used to isolate many different glycoproteins from a sample. Technical information about ConA can be found at this Vector Labs link.
WGA is short for wheat germ agglutinin. It binds preferentially to a sugar called N-acetylglucosamine, commonly found in many structures including cell membrane receptors, chitin and bacterial cell walls.
Finally, JAC is short for jacalin, isolated from jackfruit seeds. This lectin is more specific than the other two, binding only O-glycosyl-linked sugars (check your biochemistry textbook for the difference between O- and N-linked sugars). It has been used to bind the immunoglobulin IgA.

To use a lectin for purifying glycoproteins, you would employ a procedure called affinity chromatography. Basically, you take a glass column and pack it with a matrix. The matrix is made from an inert substance in the form of powder or microscopic sized beads, to which you have chemically attached your lectin (this is called conjugation). You would then wash your column with a buffer and add a cell extract containing the glycoprotein(s) of interest. The trick with any kind of column chromatography is to find the conditions (pH, buffer strength, ionic strength) which favour specific binding and minimise non-specific binding of other proteins. With luck your glycoproteins bind to the column, you wash to remove non-specific binding proteins, then elute the glycoproteins from the column using some kind of concentration gradient - typically, a compound that competes with the bound glycoprotein on the lectin. Finally you collect the eluted proteins in small fractions as they come off the column, then analyse each fraction for purity, activity and so on. I'm sure that you will learn all about protein purification as you progress through your biochemistry course. Try a Google Image search for lectin chromatography to see some pictures of the process.