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Fig. 1. The early secretory pathway. Transcripts encoding secretory/transmembrane
proteins are targeted to the ER through interaction of the N-terminal peptide
(black line), as it emerges from the ribosome (blue), with the Srp complex
(yellow oval). The Srp complex is composed of a 7S RNA and six RNA-binding
proteins, each of which is conserved in Drosophila. The Srp/signal
peptide complex interacts with the 
-subunit of the SR (brown). The
ribosome is then transferred to the sec61 complex (light blue), which is
composed of -ß-
subunits (all are conserved in
Drosophila). sec62, sec63, sec71 and sec72 are thought to be involved
in Srp-independent (or post-translational) protein translocation. TRAM is
required for efficient co-translational translocation and is important for
incorporation of transmembrane proteins into the lipid bilayer of the ER.
After the nascent peptide is translocated into the ER, the signal sequence is
cleaved by the SPC. Four out of the five peptide subunits (red lines) are
conserved in Drosophila, with one of the peptides showing homology to
two of the mammalian proteins. The COPII coatamer is involved in the
anterograde movement of vesicles from the ER to the Golgi (blue circles). It
comprises sec13, sec16, sec23, sec24 and sec31 subunits. A sec16 homolog is
not recognizable in the current annotated Drosophila genomic
sequence. Sar1 (orange circle) is a small G-protein involved in the regulation
of COPII assembly/disassembly. The p24 transmembrane family of proteins
interact with soluble cargo destined to leave the ER. COPI-coated vesicles are
involved in retrograde movement of secretory vesicles from the Golgi to the ER
(green ovals). All of the COPI coatamer components, which include 
,
, , 
-cop and ARF-1, as well as other proteins involved in
the retrieval of escaped resident proteins back to the ER are conserved in
Drosophila. The Srp region of this figure was adapted from Wild et
al. (Wild et al., 2002); the
COPII region from Shaywitz et al.
(Shaywitz et al., 1997); the
SPC region from Kalies et al. (Kalies et al., 1996); the translocon region
from Romisch (Romisch, 1999);
and the COPI region from Wieland and Harter
(Wieland and Harter,
1999).
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