Calculated MW: 11.5 kDa
Observed MW (SDS-PAGE): 15 kDa
Possible reasons why the actual band size differs from the predicted band size:
1. Splice variants. Alternative splicing may create different sized proteins from the same gene.
2. Relative charge. The composition of amino acids may affect the charge of the protein.
3. Post-translational modification. Phosphorylation, glycoslyation, methylation etc. may affect the band size.
4. Post-translational cleavage. Many proteins are synthesised as pro-proteins, and then cleaved to give the active form.
5. Polymerisation of the target protein. Dimerisation, multimerisation etc. will increase the band size observed.