CIRCULAR-DICHROISM ANALYSIS OF THE INTERACTION BETWEEN THE ALPHA-SUBUNIT AND BETA-SUBUNIT IN A KILLER TOXIN PRODUCED BY A HALOTOLERANT YEAST, PICHIA-FARINOSA

SMK toxin is a killer toxin produced by a halotolerant yeast, Pichia f arinosa, It is a heterodimer consisting of alpha (63 aa) and beta (77 aa) subunits, between which no disulfide bond exists, The two subunits interact tightly with each other below pH 5, However, the subunits di ssociate under neutral conditions, resulting in the aggregation of the alpha subunit and the concomitant loss of killer activity. CD spectra l measurements showed that the secondary structure of the SMK toxin ch anges drastically in the pH range 5.1-5.5 and that after the dissociat ion of the subunits, the soluble beta subunit alone cannot take any se condary structure. It was also shown that the concentration of NaCl do es not affect the secondary structure of the SMK toxin.