Re: what are c-terminal triple epitope tag from infuenza hemagglutinin?

This question is more a question of definitions than anything, but I'll try to add some application to clarify the response. First off, what is an epitope tag?

Antibodies, as you probably know, are small proteins produced by the immune system to identify and fight off invaders. Antibodies identify foreign proteins by binding to them through a "lock and key" interaction in which a pocket on the arm of the antibody forms a 3 dimensional mitt that exactly fits some 3 dimensional knob on the foreign protein. This interaction is very useful in protein research, because if you can raise an antibody against a protein, you can use that antibody to screen cells and tissues for the presence of that protein.

Early on in the study of immunology, the question of how antibodies interact with proteins was addressed by chopping the target proteins, called antigens, into little pieces to see if the antibodies could still recognize them and which pieces they would recognize. As it turned out, most antibodies could recognize a very small, specific fragment of their antigen, and this fragment was termed the "epitope". It was later discovered that each antibody recognizes a specific epitope, and that multiple antibodies can recognize different epitopes on the same antigen. It has since been shown that any protein that contains a given epitope will be recognized by the corresponding antibody (in fact, this can be a blessing and a curse to protein chemists: if you raise antibodies against a common epitope, you can pick up several extraneous proteins along with your initial target).

So, now we can address your question. Often, in molecular biology, we start with a gene and need to run tests on its corresponding protein. Unfortunately, keeping track of proteins - whether they are expressed, how much is expressed, etc. - is very difficult in the context of cells which contain tens of thousands of other proteins. The best way to get around this difficulty used to be to raise antibodies against the protein (provided you can isolate a pure enough sample), and use them to follow the protein. Unfortunately, making antibodies is tricky business, and some proteins are, for various reasons, nigh impossible to make antibodies against. So, some immunologists came up with a shortcut: if you can't make an antibody against your protein, why not put something on your protein that is already recognized by a specific antibody. That something is an epitope tag. Epitope tags are particularly appealing to molecular biologists, because the tag can be added at the genetic level: once you've isolated a gene that you want to express, you simply splice in a short stretch of DNA that codes for the epitope of choice; then when your protein comes out it will already have the epitope on it.

One obvious concern regarding epitope tags, is that you don't want the antibody to crossreact with other proteins. The easiest way to get around this, is to choose an epitope from a protein as far removed from your sample as possible. Since haemagglutinin (HA) is a protein specific to influenza, its epitope is not present in animals or cells that aren't suffering from an influenza infection. In fact, HA is such a popular epitope tag, that plasmid vectors are commercially available, e.g. from Clontech, that will express HA-tagged proteins in mammalian cells simply by cloning your gene into the right site. To completely answer your question, "c-terminal triple epitope tags from infuenza haemagglutinin" are epitope tags consisting of three, tandem repeats of the HA epitope placed at the carboxy-terminal of a protein (N.B., if you go to the above sites you may notice that most of their epitope tags are amino-terminal - it doesn't really matter which end of the protein the tag is on, as long as it remains visible to the antibody).