Blood, Vol. 96 No. 2 (July 15), 2000:
pp. 740-746
A novel endoproteolytic processing activity in mitochondria of
erythroid cells and the role in heme synthesis
Vijole Dzikaite,
Arvydas Kanopka,
Jeremy H. Brock,
Arunas Kazlauskas, and
Öjar Melefors
From the Microbiology and Tumorbiology Center,
Karolinska Institutet, Stockholm, Sweden, and the Department of
Immunology, University of Glasgow, Western Infirmary, Glasgow,
Scotland.
The erythroid isoform of aminolevulinate synthase (eALAS) protein is
a major control point in erythroid heme synthesis and hemoglobin
formation. Erythroid cells were extracted from mouse blood and bone
marrow and metabolically labeled with 35S-methionine. This
was followed by immunoprecipitation of eALAS protein products. The
results show that the N-terminus of the expected full-length 59-kd form
of the eALAS protein is truncated in bone marrow erythroid cells by
approximately 7 kd. More differentiated erythroid cells in the
peripheral blood exhibit very little of this protein truncation.
Erythroid cells from the bone marrow were isolated using monoclonal
antibody TER-119 and were shown to contain a unique endoprotease
activity that could cleave the eALAS protein to the shorter form in
vitro. With or without the mitochondrial signal sequence, the eALAS
protein could serve as a substrate for the cleavage. This cleavage
renders a functional eALAS protein and only removes a domain of unclear
function, which has previously been reported to vary in size as a
result of alternative RNA splicing. The protease activity was enriched
from the membranes of mitochondria from bone marrow cells and was shown
to be different from mitochondrial processing peptidase, medullasin,
and other known proteases. Apart from the mitochondrial processing
peptidase that cleaves the import signal sequence, this is the first
description of a mitochondrially located site-specific processing
protease activity.
