In cell-free systems, stress-resistant proteins nonspecifically stabilize stresssusceptible proteins. This mechanism has been suggested to contribute to therrnotolerance in cells (Minton et al.: Proc. Natl. Acad. Sci. USA, 79: 71 07-71 17,1982). To test this hypothesis, red-blood-cell-mediated rnicroinjec- tion was used to transfer macromolecules into monolayers of C H O cells. We introduced the heat-stable proteins fetuin and ovornucoid into RBCs during hypotonic hernolysis and then fused the RBCs to C H O cells with polyethylene glycol as fusogen. Fetuin and ovornucoid were successfully transferred into 36-55% of the C H O cells as demonstrated by fluorescence of FITC-conjugated proteins. The plating efficiency of these C H O cells after fusion ranged from 35% to 60%. Three hours after fusion, CHO cells microinjected with fetuin or ovornucoid were exposed to 43°C for 0-180 rnin or 45°C for 0-40 rnin, and thermal survival was determined. There was no difference in cell survival between control untreated cells, control cells fused with nonloaded RBCs, and cells fused with RBCs loaded with fetuin or ovornucoid.
While our results do not support the hypothesis that heat-stable proteins nonspecifically protect cells from thermal stress, several possible explanations are provided for this observation.