The Synthesis of the Immobilization Antigen in Paramecium aurelia: In vitro Localization of Antigen in Ribosomal Cell Fractions

SYNOPSIS. The distribution of a cell surface protein, the immobilization antigen, has been studied in homogenates of Paramecium aurelia stock 168, syngen 1. Two fractions, a 9,000 g supernatant fluid and a 25,000 g supernatant fluid, have been investigated for their ability to bind 125I‐labelled antibodies specific against immobilization antigen. These 2 fractions have been characterized by electron microscopy. Antigen was detected in a pure microsome fraction and in the polysome fraction. Sucrose gradient analysis of the polysome fractions indicates that antigen is associated specifically with polysomes of 228S and 311S, the latter being located primarily in the microsome fraction. These results correlate well with the structure of the immobilization antigen proposed by Steers. A possible mechanism for the synthesis of this protein is outlined.