SDS-PAGE fractionation of acid-soluble (ASM) and acid-insoluble (AIM) shell matrix proteins. The mass of molecular weight markers (in kDa) is indicated on the left. Following electrophoresis under denaturing condition, proteins were stained with CBB. Approximately 80 μg of protein material were applied per well. I1-I12 and S1-S8 correspond to bands that were excised for in-gel digestion and MS/MS analysis.