Fusion protein ativity

[KHUANG at CHEMVX.TAMU.EDU]

Hi, there,
I am using GST and thioredoxin fusion to express a few plant protein, the
molecular weight are more than 60 KDa. I tried four, three are activity, one
did not show activity, but when the protein was released from fusion protein, 
it showed some activity. I want to know whether you also have these experience,
I saw some paper they reported the released protein show 18-20 higher activity
than the fusion protein. Is it usual?
Thanks!

Huang