Volume 270,
Number 22,
Issue of June 2, pp. 13503-13511, 1995
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
EEA1, an Early
Endosome-Associated Protein
EEA1 IS A CONSERVED
-HELICAL PERIPHERAL MEMBRANE PROTEIN
FLANKED BY CYSTEINE ``FINGERS'' AND CONTAINS A
CALMODULIN-BINDING IQ MOTIF
Fi-Tjen
Mu
,
Judy
M.
Callaghan
,
Olivia
Steele-Mortimer
,
Harald
Stenmark
,
Robert
G.
Parton
,
Paul L.
Campbell
,
James
McCluskey
,
Jing-Ping
Yeo
,
Edward P.C.
Tock
,
Ban-Hock
Toh
Early endosomes are cellular compartments receiving endocytosed
material and sorting them for vesicular transport to late endosomes and
lysosomes or for recycling to the plasma membrane. We have cloned a
human cDNA encoding an evolutionarily conserved 180-kDa protein on
early endosomes named EEA1 (Early Endosome
Antigen1). EEA1 is associated with early endosomes since it
co-localizes by immunofluorescence with the transferrin receptor and
with Rab5 but not with Rab7. Immunoelectron microscopy shows that it is
associated with tubulovesicular early endosomes containing internalized
bovine serum albumin-gold. EEA1 is a hydrophilic peripheral membrane
protein present in cytosol and membrane fractions. It partitions in the
aqueous phase after Triton X-114 solubilization and is extracted from
membranes by 0.3 M NaCl. It is a predominantly
-helical
protein sharing 17-20% sequence identity with the myosins and
contains a calmodulin-binding IQ motif. It is flanked by metal-binding,
cysteine ``finger'' motifs. The COOH-terminal fingers,
Cys-X
-Cys-X
-Cys-X
-Cys
and
Cys-X
-Cys-X
-Cys-X
-Cys,
are present within a region that is strikingly homologous with
Saccharomyces cerevisiae FAB1 protein required for endocytosis
and with Caenorhabditis elegans ZK632. These fingers also show
limited conservation with S. cerevisiae VAC1, Vps11, and
Vps18p proteins implicated in vacuolar transport. We propose
that EEA1 is required for vesicular transport of proteins through early
endosomes and that its finger motifs are required for this activity.