is it the same protein

[Caroline J. Walker]

Can you take your high mw fraction off the gel filtration column,
concentrate it and run it through again - see if you generate lower mw
monomers from it i.e. you may be able to promote some futher dissociation
of the aggregated form.  I got a similar kind of result with a protein, and
found that the Mg concentration was critical in determining the level of
aggregation (in its absence it ran as monomers).
	 The other possibility you may consider is that in crude preps it
is possible that your protein may be associating with other proteins rather
than just itself e.g. if its a part of a metabolon or other large complex
(even if this association is loose).