Unexpected migration. Some proteins migrate anoma- lously in SDS-PAGE gels. For instance, the molecular weight determined from an SDS-PAGE gel is sometimes very differ- ent from the molecular weight determined from the amino acid sequence. Suggest an explanation for this discrepancy.

Glycosylation, Phosphorylation, Proteases etc

Explanation:

Difference in molecular weight of protein on SDS-PAGE may vary to the calculated amino acid weight due to the following reasons

1. GLYCOSYLATION: Most ribosomal proteins undergo glycosylation,  with a covalent attachment of sugar societies to the peptide chain. This attachment accounts for an increased weight and eventual slow migration on SDS-PAGE gels giving rise to discrepancy between gel molecular weight and calculated molecular weight

2. PHOSPHORYLATION: This is one of the most common molecular changes that occurs after translation. It Is caused either by enzymatic activity,  phosphatases, or regulating proteins.

Other factors which may affect migration on electrophoresis may be presence of proteases, protein complexes or incomplete unfolding of protein