I’m encountering a problem detecting an HA epitope tag that I introduced via cloning into my protein of interest. When performing Western blots on lysates from HEK293T cells transfected with my expression construct, I cannot detect the HA tag, even though my protein itself is clearly present.
It is a Drosophilaprotein I am trying to express in a mammaliancell line. I have not done codon optimization, however I have other fusions (RFP-, GFP-) that express just fine.
-It is NOT secreted, nor does it have a signalingsequence at its N-terminus that would post-translationally be cleaved.
Any ideas out there? Is there something I’m missing about the construction of my fusion? Is there some detail about HA-tags I might be overlooking?
This is a very common problem with small epitope tags, and the fact that you see the protein but not the HA signal actually narrows things down nicely. Below is a structured way to think through it, from most likely to more subtle causes.
Failure to detect an HA tag despite clear protein expression is usually due to epitope inaccessibility, since the HA tag is very small and can be buried by protein folding or interactions. This is especially likely if larger tags (GFP/RFP) work. Adding a flexible linker or moving the HA tag to the opposite terminus often resolves the issue. Proteolytic cleavage of terminal HA tags is another possibility, even for non-secreted proteins. Antibody-related problems are common—different anti-HA clones vary greatly in Western blot performance, so testing alternatives and including a positive control is essential. Less common causes include insufficient denaturation, harsh sample prep, or cloning junction/frame errors. Codon optimization is unlikely to be the problem.
The HA tag is only 9 aa long (YPYDVPDYA). If it is:
buried inside the folded protein
masked by protein–protein interactions
close to a structured domain or membrane
constrained by the fusion junction
then antibodies simply can’t bind it—even on a denaturing Western.