Auxin Effects on the Aggregation and Heat Coagulability of Cytoplasmic Proteins and Lipoproteins

The kinetics of induction of heat stability of cytoplasmic proteins and lipoproteins by auxin (2,4,‐D) were determined for basal sections of soybean hypocotyl. Maximum heat stabilization occurred after 4 h of tissue incubation with 10 ‐5 M 2,4‐D. The effect was less pronounced or absent with longer incubations. Membrane fractions sedimenting between 10,000 and 100,000 g and proteins of the 100,000 g supernatant were most affected. The auxin‐induced protein aggregation response varied among experiments. With many tissue lots, the response was small or absent even though the tissue responded to the auxin uniformly by increased growth. The magnitude of response was proportional to the logarithm of auxin concentration but with low 2,4‐D the portion of the homogenate protein coagulated by heat was increased and with supraoptimal concentrations it was decreased relative to the control. The smallest auxin‐induced change in heat coagulability was observed at the auxin concentration nearest the optimum for growth. No direct correlation was found between the auxin‐induced protein and lipo‐protein aggregation phenomenon and total protein, chloroform‐extractable lipid, residual lipid, growth or tissue deformability. Total sulfhydryl equivalent of the homogenates, however, did correlate with auxin effects on aggregation. This result, plus experiments where homogenates were exposed to oxidizing or reducing conditions, suggests that heat stabilization and associated protein aggregation phenomena are related to conversion of protein sulfhydryl to intramolecular disulfide bonds. No significance is attached to heat stabilization of cytoplasmic proteins as a requisite of auxin‐induced growth.