The liquid protein phase in crystallization: a case study - intact immunoglobulins

A common observation by protein chemists has been the appearance, for many proteins in aqueous solutions, of oil like droplets, or in more extreme cas es the formation of a second oil like phase. These may accompany the format ion of precipitate in "salting out" or "salting in' procedures, but more co mmonly appear in place of any precipitate. Such phase separations also occu r, with even greater frequency, in the presence of polymeric precipitants s uch as polyethyleneglycol (PEG). In general the appearance of a second liqu id phase has been taken as indicative of protein aggregation, though an agg regate state distinctly different from that characteristic of amorphous pre cipitate. While the latter is thought to be composed of linear and branched assemblies, polymers of a sort, the oil phase suggests a more compact, thr ee-dimensional, but fluid state. An important property of an alternate, flu id phase is that it can mediate transitions between other states, for examp le, between protein molecules free in solution and protein molecules immobi lized in amorphous precipitate or crystals. The "liquid protein" phase can be readily observed in many crystallization experiments either prior to the appearance of visible crystals, or directly participating in the crystal g rowth process. In some cases the relationship between the liquid phase and developing crystals is intimate. Crystals grow directly from the liquid pha se, or appear only after the visible formation of the liquid phase. We desc ribe here our experience with a class of macromolecules, immunoglobulins, a nd particularly IDEC-151, an IgG specific for CD4 on human lymphocytes. Thi s protein has been crystallized from a Jeffamine-LiSO4 mother liquor and, i ts crystallization illustrates many of the features associated with the liq uid protein, or protein rich phase. (C) 2001 Elsevier Science B.V. All righ ts reserved.