Protein expression help needed - Mammalian cells (Dec/03/2007 )
generally optimem is better for cell transfection. Also you may choose the plus reagent if you can't afford the lipofectamine 2000 solution. It greatly enhanced my tranfection efficiency of endothelial cells and IMR 90 (fibroblast primary cell line). Ok in know these aren't vero, but tough and sensitive regarding transfection.
Considering DMEM, you may also consider the low glucose formulation. IMR90 goes better in hat one, as well for transfection.
For some difficult-to-express target proteins with some particular cell lines, it is no help even by optimilizing of all kinds of conditions. Why not generate stable cell lines? I have resolved all lower expresion problems for lots of difficult-to-express target genes, including ion channels, transporters, GPCR, nulcear receptors and cross-species chimeras. Certainly, the suitable vector for stable cell line generation is also very important. I do not recommend Inivitrogen's vector, try vectors from Clontech. If your problem is still there, I might be able to help in this regard.