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Origin of a soluble truncated transferrin receptor
J Ahn and RM Johnstone
Department of Biochemistry, McGill University, Montreal, Quebec, Canada.
It has recently become evident that elevation of reticulocytes in the
circulation of several species, including humans, leads to the formation of
a noncellular transferrin receptor (TFR). In humans, the majority of the
released receptor is in truncated form (Shih et al: J Biol Chem 265:19077,
1990). In other species (sheep, rat, chicken) the receptor is associated
with a vesicle (exosome) and is full length (Johnstone et al: J Cell
Physiol 147:27, 1991). In this report we show that in sheep reticulocytes
incubated in vitro, the majority (approximately 75%) of the released
receptor is of native size and is exosome associated. A fraction
(approximately 25%) is a truncated form of approximately 80 Kd
corresponding to the exofacial domain of the TFR. Herein we also address
the question of whether the truncated receptor originates by proteolytic
cleavage directly from the cell surface or by cleavage from exosomes. Using
surface 125I-labeled sheep reticulocytes as the experimental model, we show
that during in vitro maturation, 125I-TFR of native size appears in
exosomes before the soluble, truncated, exofacial domain of the receptor is
detected in the medium. Because cleavage and release of the exofacial
domain would likely leave the truncated cytoplasmic and transmembrane
domains in the originating membrane (plasma membranes or exosomes), both
fractions were probed with antibodies specifically generated against the
cytoplasmic domain of the receptor. Only exosomes, not plasma membranes,
show the presence of a approximately 17-Kd peptide recognized by the
antibody to the cytoplasmic domain of the transferrin receptor. Thus, it is
concluded that the truncated, soluble receptor originates from exosomes in
sheep. A 17-Kd cytoplasmic domain of the TFR was also detected in exosomes
from the reticulocytes of an anemic man, suggesting that the truncated
receptor in man may also originate from exosomes. Using in vitro cultures
of surface 125I-labeled sheep reticulocytes, it is concluded that exosome
formation is the principal route for maturation-associated loss of the TFR.
A similar conclusion was made earlier (Johnstone et al: J Cell Physiol
147:27, 1991) for the nucleoside transporter of maturing sheep
reticulocytes.
Volume 81,
Issue 9,
pp. 2442-2451,
05/01/1993
Copyright © 1993 by The American Society of Hematology
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