Hi there,

The formation of the tetramer occurs during the crystallization process but it has no biological relevance if you can't characterize it in solution.

This could be a concentration dependent artifact. Consider that a crystal of a protein is an ultra high concentration of protein - even though you set up the condition at say 10mg/ml the amount of protein molecules in a crystal (depending on size of the crystal and protein) could be >500mg/ml (or >50mM). If the crystallographic tetramer is symmetrical it is possible that the quaternary structure is real, but concentration or pH dependent. You can submit your monomer to the PISA server to calculate the probability of multimerization also. 

You can easily experimentally assess this phenomena with size-exclusion chromatography at different concentrations and pH as well as dynamic light scattering and/or Small Angle Xray Scattering. Regarding SEC, make sure you choose a column that is capable of resolving a tetramer and monomer for your studies. 

Also, you didn't mention the crystallographic condition (salt and concentration, etc), but salting out is something to consider when dealing with proteins in solution. If the condition includes 1M Ammonium Sulfate, then the nucleation could start with salting out the protein (for instance). 

What you see in the crystal  is not necessarily  what is present in solution.As a first

approximation one usually assumes  that the biomolecular structure  is similar in

@carter  (see  also my uploaded papers with Yannis Georgalis  et al  concerning

microaggregation  & crystallization  in simplest systems  like lysozyme NaCl  Water

let alone the exotic multicomponent  coctails used by crystallographers to enforce

crystallization).I think we do not understand enough to handle the aggregation  phenomena in these systems  in a predictive manner.