Tyrosine sulfation is not the last modification of entactin before its secretion from 3T3‐L1 adipocytes

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Abstract
Tyrosine sulfation of entactin was studied by labeling of 3T3‐L1 adipocytes with [35S]methionine or H2 35SO4 in the presence or absence of tunicamycin or monensin. Four precursors (EN1–4) at different steps of modification were detected in addition to mature entactin. Under normal conditions, EN2 and mature entactin were intracellular species, and the latter was sulfated and secreted. Inhibition of co‐translational transfer of N‐linked oligosaccharides by tunicamycin produced EN1 and EN3 as intracellular species, and EN3 was sulfated and secreted. Interruption of protein transport from medial to trans (distal) Golgi cisternae by monensin, and consequent blockage of terminal glycosylation caused intracellular accumulation of EN4. EN4 was sulfated and of different size compared to mature entactin. These facts suggested that tyrosine sulfation of entactin occurs in medial Golgi cisternae and is not the last modification before its secretion. Our results appeared inconsistent with recent observations by Baeuerle and Huttner [(1987) J. Cell Biol. 105, 2655–2664] that tyrosine sulfation of IgM occurred within the trans (distal) Golgi cisternae as the last modification before its exit from the Golgi complex.