Question

You are working on the protein α-catenin in C. elegans and have produced a monoclonal antibody that recognizes this protein. The normal protein identified by this antibody on a Western blot is 104kDa. You go on to identify a mutant that you think produces no protein at all (geneticist call such mutants “protein null” mutants) because Western blots performed using your antibody with protein from this mutant yield no signal. Later, however, another member of your research group sequences your mutant’s DNA and finds that it contains a nonsense codon that produces an early stop codon. Puzzled by this, he uses a polyclonal antibody to perform another Western blot and finds that a band of 89kDA is visible. How can you explain your labmate’s results?

Answer 1

Answer:

Monoclonal antibodies bind to a single epitope within a target antigen, while a polyclonal antibody represents a collection of antibodies from different B cells that recognize multiple epitopes on the same antigen.

The wild type $\alpha$ -catenin gives a band at 104 kDa in western blot when monoclonal antibody towards the protein is used. Again, the mutant cannot be detected in western blot using the monoclonal antibody, but a band is visible at 89 kDa with the polyclonal antibody.

Thus, it can be inferrred that:

1. It is a truncated version of the protein which does not contain the epitope recognized by the monoclonal antibody, but has the epitopes recognized by the polyclonal antibodies.

2. It is a completely different protein (a contaminant) which shows up due to non specific binding of the polyclonal antibody and the mutant protein is not produced at all.