P53 protein normally functions in the nucleus by responding to DNA damage or appopriate cell signals to arrest cell growth or participates in programmed cell death if damage is irreparable. If p53 gene becomes mutated, the resulting gene products may assume an aberrant conformation incapable of negative growth control but newly capable of binding with other proteins like HSP70. In human and murine cells expressing aberrant conformation p53, we have identified members of p53-HSP complexes which sequester p53 to the cytoplasm and include GRP75, GRP78 and a weakly basic 90 kD protein. Isolation from native lysates has shown cytoplasmic HSP complexes exist as pre-formed multimers which may be important for recognition and binding of aberrant p53. The presence of GRP75 and GRP78 suggested p53 might be found within these organelle-resident HSP's which was confirmed for GRP75 in mitochondria by biochemical and immunoelectron microscopic analyses. More recent work on GRP75 shows it normally exists in 4 to 5 charge separable isoforms which are increased in number by glucose deprivation but not thermal stress. An additional important aspect to this work is the patented preparative 2D PAGE system developed for isolating and sequencing proteins which was used to identify HSP proteins in this work. The overlying hypothesis of this project is that binding of these organelle-resident HSP70 members to mutant p53 reflects a general cellular response to accumulation of an abnormal protein (p53) by invoking a specific chaperone program for binding and processing. The nature of such processing and the ultimate fate of HSP-bound p53 is not yet understood but degradory pathways leading to enzymatic cleavage and formation of peptides for antigen presentation are being examined as likely areas for future studies.
