A double-isotope labeling approach has been employed in an attempt to identify the proteins synthesized by lymphocytes early after stimulation by phytohemagglutinin (PHA). The earliest effect of PHA, within the first hour, was the induction of large aggregates of cellular proteins, which were not dissociated by 1% sodium dodecyl sulfate (SDS) in the absence of β-mercaptoethanol. These aggregates were composed of proteins of molecular weight approximately 70,000, but they did not include PHA. The aggregates were made up of preexisting as well as newly synthesized cellular proteins. Subsequently, within the first 2 hr after the addition of PHA, there was a nonspecific stimulation of protein synthesis. This was followed by the preferential synthesis of several classes of proteins including at least one group of nuclear proteins. The structural changes described here are among the earliest events known to occur within the lymphoid cell after its interaction with PHA.
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