TRANSIENT ER RETENTION AS STRESS-RESPONSE - CONFORMATIONAL REPAIR OF HEAT-DAMAGED PROTEINS TO SECRETION-COMPETENT STRUCTURES

Mechanisms to acquire tolerance against heat, an important environment al stress condition, have evolved in all organisms, but are largely un known, When Saccharomyces cerevisiae cells are pre-conditioned at 37 d egrees C, they survive an otherwise lethal exposure to 48-50 degrees C , and form colonies at 24 degrees C, We show here that incubation of y east cells at 48-50 degrees C, after pre-conditioning at 37 degrees C, resulted in inactivation of exocytosis, and in conformational damage and loss of transport competence of proteins residing in the endoplasm ic reticulum (ER), Soon after return of the cells to 24 degrees C, mem brane traffic was resumed, but cell wall invertase, vacuolar carboxype ptidase Y and a secretory beta-lactamase fusion protein remained in th e ER for different times, Thereafter their transport competence was re sumed very slowly with widely varying kinetics, While the proteins wer e undergoing conformational repair in the ER, their native counterpart s, synthesized after shift of the cells to 24 degrees C, folded normal ly, by-passed the heat-affected copies and exited rapidly the ER, The Hsp70 homolog Lhs1p was required for acquisition of secretion competen ce of heat-damaged proteins, ER retention and refolding of heat-denatu red glycoproteins appear to be part of the cellular stress response.