Hi Laura

The 150kda band is most probably the full antibody (that's why as this gets stronger the heavy and light chains are getting weaker). It is obvious that the problem is sample reduction/denaturation so I would recommend checking your reducing agent, heating temperature of your sample and your sample buffer in general. The chances are that your IP actually works but your western blot doesn't. This could be due to the fact that your antibody for your target of interest can only recognize the antigen/epitope when the latter is fully reduced.

Kind regards

Thanks very much Nikolaos, It had occurred to me that the full antibody would be this size. However I am using the same reducing sample buffer as I had in the past and heating at 95 degrees for 3 minutes so I thought that should have been enough to denature! When I repeat I will change my reducing buffer and perhaps heat longer, it just seems surprising to me that a protein could survive that treatment! 

Heating is not going to reduce your antibody, as it does not break covalent disulfide bonds. Rather, your reducing sample buffer may have gone bad. I, personally, prepare my reducing sample buffer freshly before every experiment from non-reducing sample buffer to which I add DTT from a frozen stock.

If your starting material is a cell lysate, it might be a good idea to load a lysate control on the gel, so that you have a positive control (you know your 120 kDa protein should be in that lane). If your antibody detects your protein in the lysate but not in the co-IP then something with your IP is wrong. However, if the antibody detects the protein neither in the lysate nor in the co-IP then your Western is probably not working.