You've probably sorted out the problem by now but Invitrogen also
offers trc expression vectors based on its topo recombination
technology for quick cloning.
The manual offers trouble shooting advive on page 18 which could
equally apply to your expression vector.
http://www.invitrogen.com/content/sfs/manuals/ptrchistopo_man.pdf
Are you sure when you sequenced your clone the insert was is in frame
with the initiation codon?
