The C2C12 lysate is supplied in buffer that consists nonionic and ionic detergents, including 0.2% SDS. If you are running denatured gels, addition of one or a few phosphates isn't likely to change the molecular weight. Try a gel without SDS, charge difference will separate unphosphorylated forn from phosphorylated version quite nicely. A better but slightly tedious method is to perform a 2-D gel.
If you can find an antibody that specifically recognized phosphorylated InsR, that would be helpful.
Hi Revathy, I donn't known very well the insulin pathway, but, even if Neus point out an important "technical" aspect (your antibodies are new, they were used by someone else ?), I think tha you have also to check the basic expression of this receptor from literature. Did the receptor is normally expressed in C2C12? Is it expressed in myoblast and in myotube? How much is expressed, maybe you have to load a lot more lysate? For the phosphorylated one, I think you treated the cells (for example ith insulin), did you think that the experiment (treatment) work well? Maybe the treatment failed, for this reaon you din't see the phospho-receptor? If you can exclude technical problems read some literature to understand why you don't see anything.