Dear Romain Gioia

Since WB is a denaturing approach is quite strange that a mab recognize well the recombinant and not endogenous protein if the protein sequence is the same.

Or do you have a quantitative issue due to a low amount of endogenuos protein expressed from the cell or recovered during the sample preparation (e.g some times membrane proteins are not extracted with high efficiency and after cell lysis remain in the insoluble fraction and are not loaded in the WB)

or there is some difference in terms of sequence (eg different isoforms?) beetween your protein and the recombinant one, but since you are using a policlonal and not monoclonal, i suspect that it will be able to recognize also protein with some differences.

good luck

Manuele

It depends on the antigen that was used for the protein detection. Was the recombinant protein used to make the antibody? It also could be that your endogenous protein is not expressed at a high level and the recombinant protein is at a high concentration.

Dear @Romain Gioia, this could happen if the first polyAbs are against the peptide from the whole target protein. In WB an endogenous protein may mask this epitope/peptide. The other reasons are already mentioned.