Distribution of concanavalin A in fibroblasts: direct endocytosis versus surface capping.

Indirect immunofluorescence of intact or acetone-extracted cells has allowed us to distinguish concanavalin A (Con A) which is associated with the plasma membrane of CHO cells from Con A which has been interiorized. We find that Con A is directly endocytized by these cells with no intervening stage of plasma membrane aggregation. The lectin accumulations observed by direct fluorescence are actually cytoplasmic collections of pinosomes which contain Con A. Only in a small fraction of CHO cells are true plasma membrane aggregates, or caps, found. This predominance of direct pinocytic interiorization over capping was not affected by dibutyryl cAMP or by treatments which can disrupt microtubules, including cold shock or exposure of the cells to anti-mitotic agents. Cytochalasin B, however, inhibited the uptake of Con A and at the same time promoted the formation of large surface aggregates of the lectin, or minicaps. Capping may reflect a competition between aggregation in the plane of the membrane and direct interiorization of bound lectin. Surface cap formation may be a characteristic process of cells with very low endocytic rates, such as lymphocytes.