Recently one of my friends with name of Dr Zilong Sun from Shanxi agricultural university, Shanxi, China, wrote an article in this area, but this manuscript is unpublished now, then I cant send you his method, unfortunately, you can contact him for this method, if you couldn't find him, please tell me. I will call him for you and after his allowance, I will send you immediately.
But you can search about this topic if you are in hurry:
After extraction of proteins by his method, protein samples will be loaded on the 12% SDS-PAGE gel followed by a 12 h trypsin digestion at 37°C (trypsin: protein=1:20).