Please help: A proteolyzed product running bigger on an SDS gel?

[cv2135 from columbia.edu]

Hello
I'm trying to get to the bottom of a claim in my lab that  a breakdown  
product of our favorite protein actually runs at a HIGHER molecular  
weight than the full-length product on a 10% SDS gel.
The breakdown product is supposedly 13 residues shorter at the C-terminus.


Our protein is:

-12.7 KD protein,

-very basic (with a PI of 9.42)

-binds RNA

  -supposedly unstructured in the absence of its specific RNA ligand  
(not generally present within E.coli)


Our protein runs at around 13-14 KD, and the supposed breakdown  
product at 15KD

For those of you interested, the full-length  protein sequence can be  
found at :

http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?db=protein&id=139123

(note first two residues, ML, represent a false start, and the NTD  
methionine is cleaved in vivo, thus the length of the protein is  
actually 108 AA)


Please , I'd appreciate it if anyone who can help me to understand an  
explanation for  this claim

Thanks so much
CV