1) If a protein undergoes post-translational modification, for example, glycosylation, the molecular weight observed on a SDS-PAGE gel will be a sum of the protein mass and the modifier mass.
2) If a protein is expressed as a proprotein, i.e. including a propeptide, the protein will undergo maturation (having the propeptide removed). Only the matured protein will be purified and analyzed by SDS-PAGE. Therefore what observed from an SDS-PAGE gel is the molecular weight of a matured protein. If the biological active form of your protein is an oligomer, i.e. containing more than one copy of a protein, the protein will maintain as an oligomer under a non-reducing condition. Therefore the molecular weight shown on an SDS-PAGE gel corresponds to the protein oligomer.
