PRODUCTION OF THE SERUM FORM OF THE TRANSFERRIN RECEPTOR BY A CELL MEMBRANE-ASSOCIATED SERINE-PROTEASE

Recent investigations have demonstrated that the soluble form of the t ransferrin receptor in human serum is an 85-kDa fragment of intact rec eptor containing most of the extracellular domain. The recent demonstr ation of a remnant of the truncated receptor in cell membranes suggest s that the soluble form arises from proteolytic cleavage of intact rec eptor. In the present investigation, domain-specific antibodies were u sed to further examine the subcellular location and nature of this pro teolysis. HL60 cells were used in the investigation because the cells release an 85-kDa soluble form of the receptor to the culture supernat ant that is identical to that found in serum. When intact, purified tr ansferrin receptor from human placenta was incubated with the culture supernatant, no proteolytic activity could be demonstrated. However, w hen purified membrane fractions from HL60 cells were used in this incu bation system, the 85-kDa fragment was produced. This activity was inh ibited by serine protease inhibitors indicating that cell membrane fra ctions contain a serine protease capable of producing the serum form o f the transferrin receptor.