Please help me :Protein expression and purification - (Jun/25/2014 )

Hi everyone， I am doing a protein expression and purification by E.Coli and some vector PET system,but there are many problems during my experiment.First,I should tell you something about my protein:400-500bp, cysteine rich protein and The repeat amino acid sequence such as AAAAAAA、LLLL 、TTT、RRR and so on.I use PET15b 、 PET28a and other PET Vectors cloned in BL21(DE3),However I can not find my Target protein through E.Coli culture through Ni-NTA purification by SDS-PAGE gel.Now I am very confused How and when I can get my target protein.Please help me!!!(ps:it is a member of LYNX，I am very thanks for you if you can give some articles about it)

maybe you could look at the fermentation conditions especially the induction temperature cuz maybe the protein is expressed as an inclusion body which u can make sure about by SDS-PAGE of 1/OD samples during the fermentation 

i'm using pET28a with an induction temperature 28 C (induction with IPTG 1 mM)