Restricted localization of the adipocyte/muscle glucose transporter species to a cell surface‐derived vesicle fraction of 3T3‐L1 adipocytes

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Abstract
The recent demonstration of a large cell surface‐derived pool of insulin‐sensitive glucose transporters, presumably concentrated in the microvilli of 3T3‐L1 adipocytes, induced the assumption that in differentiated adipocytes, newly inserted plasma membrane areas may display restricted lateral mobility, thereby preventing diffusion of integral membrane proteins out of these areas into the adjoining plasma membrane. In order to test this assumption, the cell surface distributions of the two glucose transporter species expressed by 3T3‐L1 cells were determined using specific antisera against the HepG2/erythrocyte transporter, GluT1, which is synthesized in both fibroblasts and adipocytes, and the adipocyte/muscle‐specific transporter, GluT4, expressed for the first time 3–4 days after induction of adipose conversion. GluT1 was shown to be localized in the plasma membrane of both 3T3‐L1 preadipocytes and adipocytes, whereas GluT4 was almost entirely restricted to the low density surface‐derived vesicle (LDSV) fraction of 3T3‐L1 adipocytes most likely consisting of microvilli‐derived vesicles. In contrast to the minor portion of GluT4 found in the adipocyte plasma membrane fraction, equal amounts of the GluT1 protein were detected in both the plasma membrane and the LDSV fractions of adipocytes. Both transporter species were present in the microsomal and the LDSV fractions of adipocytes. The observed distribution of the two transporter species is in accordance with the postulated restriction of the lateral mobility in plasma membrane areas formed by newly inserted transgolgi vesicles of differentiated adipocytes.

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