Protein TAG for affinity purification - (Aug/20/2010 )

Hi,
I'd like to express proteins and purify them by affinity chromatography, but i don't really know what tag to use. Actually, i would like to tests 4 or 5 different tags, like His-tag, MBP-tag, etc... I've heard of a tag which would be autocleavable in presence of ubiquitin ? Does anyone have any suggestion, any idea on tags working better than others ? My aim is to cristallize those proteins so purification by IP is not the best way to do... so i have to do without FLAG, HA, Myc, V5 or i don't what tags...

Thanks for you help !
Morgan

His tag is commonly used for purification of proteins using NiNTA beads.