I'm trying to transfect N and P proteins of PPR virus in mammalian expression system. Why can't the proteins be viewed on sds page, why is western blotting necessary to observe the expressed protein?
It is necessary, first of all, because of handling, it would be hard to handle the thin gels. However the main reason is, that because of the mechanical properties of the Gel, it would be pretty hard, to do staining, especially with antibodies.
as well as Jan's excellent answer sensitivity is a reason. Antibody staining is more sensitive than chemical staining so you can see smaller amounts of protein.Also the transfer of protein from a thick gel onto the surface of the membrane concentrates the protein making a quicker reaction with no sample diffusion and is more economical on reagents while giving a tight visible band which would be more diffuse in the gel matrix