Negative staining with Ponceau red

[GWDGV1::KCHOWDH]

Hi everybody,
This is a question from a colleague of mine.  She electrotransferred some protein bands from a SDS-
PAA gel onto nitrocellulose paper. After staining the paper with ponceau red she observed that the 
paper was stained red and the regions containing the protein bands remained white. Does anybody know 
of any explanation for this ?   Kamal Chowdhury