what is the best kit available for purifying proteins with his-tag expressed in 293 cells??
There may not be one particular kit that works best for all proteins but rather many kits could work and the best process may lie rather in the art in which the operation is performed. The metal choice, processing speed, scale, and cost may also be determining factors Qiagen Ni -NTA Superflow is popular and may be a good choice because the guidelines are well described. His60 Ni-IDA may be another popular choice but again it may not matter if your expression level or chromatography conditions are not adequate. When issues occur they consistently seem to be the same regardless of the kit: no binding, no elution, elution but with contaminants, etc. The usual solution seems to involve the expression system and/or the protein itself or the running conditions and even the molecular biology.
Kits are nice sometimes, but using them as a "black box" and believe in some magic properties when the box it comes in is looking nice will not do the job. There are two basic chemistries, NTA and IDA resins and there you go. If the standard procedure does not work well, you'll have to play with the parameters. IMHO it is important to understand how IMAC works and which factors influence the outcome, as Grant already has pointed out.
For a rather comprehensive description of IMAC read here: http://www.cube-biotech.com/s-products/purification-resins/his-affinity-resins
Do not rely upon the His-tag purification for complete purification, since there will be other proteins in mammalian cells that will have affinity for the immobilized metal resin. Be prepared to perform at least one more chromatography step.
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