COMMENTARY
Reticuloplasmins: a novel group of proteins in the endoplasmic
reticulum
G. L. E. KOCH
Laboratory of Molecular Biology, Medical Research Council, Hills Road, Cambridge, CB2 2QH, UK
The existence of a special milieu within the endoplasmic reticulum (ER) has been well known to microscopists because of its special osmiphilic properties and
the term 'reticuloplasm' was coined to describe the
contents of the ER (Krstic, 1979). The purpose of this
article is to bring together some recent observations
showing that reticuloplasm does indeed contain a very
special complement of its own resident proteins,
referred to as reticuloplasmins, and to consider the
possible role of these proteins in ER structure and
function.
The formal identification of a major non-integral
membrane protein uniquely localized to the lumen of
the ER, was effected with the protein called endoplasmin (Koch et al. 1986). The protein was shown, by
immunocytochemical studies, to be present in the ER
and undetectable in other membrane compartments.
Immunofluorescence studies have also confirmed the
findings from immunoelectron microscopy. Biochemical analyses are possible with endoplasmin because it is
a glycoprotein (Koch et al. 1985). Thus the glycosyl
moiety is completely sensitive to endoglycosidase H,
suggesting the high-mannose composition expected for
an ER glycoprotein (Koch et al. 1986). This has now
been formally confirmed with the pure glycoprotein,
which does indeed contain the Man 9 GlcNAc2 composition of an ER-specific high-mannose oligosaccharide.
Thus both immunolocalization and biochemical tests
confirm the ER location of endoplasmin.
That endoplasmin is a luminal component of the ER
is evident from several observations. It can be extracted
from cells without using membrane-solubilizing agents
such as detergent, and can be purified as a soluble
dimeric globular protein without detergents. The purified protein contains the same N- and C-terminal
sequences as that predicted for the intact molecule
from the DNA sequence and the application of the
Triton X114 test (Bordier, 1981) to the protein in cell
extracts or to the pure, intact protein exclude the
Journal of Cell Science 87, 491-492 (1987)
Printed in Great Britain © The Company of Biologists Limited 1987
presence of a post-translational hydrophobic modification such as that in the VSG protein. Thus there
seems little doubt that endoplasmin is a luminal
component of the ER, i.e. a reticuloplasmin.
Several other candidate reticuloplasmins have been
identified. One of the best-known is the enzyme protein
disulphide isomerase or PDI. It is clearly a globular
protein present in substantial amounts in microsomal
membrane preparations from secretory cells (Freedman, 1984). Another major constituent in such preparations, apart from endoplasmin, is the protein called
BIP (Bole et al. 1986), which appears to bind immunoglobulin heavy chains in B-cells. In this context it is
worth mentioning that a study on purified microsomal
membranes from plasmacytoma cells has indeed shown
that the three major polypeptides in such preparations
have molecular weights of lOOxlO3 (endoplasmin?),
75X103 (BIP?) and 60xl0 3 (PDI?), respectively
(Lewis et al. 1985).
The first implication of these observations is that it is
no longer appropriate to represent the lumen of the ER
as a relatively empty space containing secretory products. Current estimates suggest that the protein concentration approaches that in other compartments such
as the cytosol.
The other implication is that there must be a number
of unidentified ER functions requiring these proteins.
In formal terms, the only protein with a known
function is PDI, which is almost certainly involved in
the formation of disulphide bonds in the ER (Freedman, 1984). A striking feature of several reticuloplasmins, notably endoplasmin, PDI and a 55xlO3.A'/r
protein is that they bind significant amounts of calcium
(Koch et al. 1986). Since the ER does not appear to
contain a single abundant calcium storage protein such
as the calsequestrin in sarcoplasmic reticulum, the
putative calcium storage role of the ER (Streb et al.
1983) could be effected by the above-mentioned reticuloplasmins. The fact that several of these proteins
can be specifically induced by calcium ionophores
491
(Welch et al. 1983; Wu et al. 1981; Koch & Macer,
unpublished observations) supports such a possibility.
One other possible role for some of the reticuloplasmins is based on the observation that they are highly
conserved in their amino acid sequences (Smith &
Koch, 1987). A high level of sequence conservation is
usually associated with proteins such as actin, tubulin
and the histones that perform a scaffolding or structural role. The increased levels of endoplasmin and some
other ER proteins upon exposure of cells to stress (Lee
et al. 1984) would be consistent with such a role since
supramolecular assemblies such as the cytoskeleton are
known to be particularly sensitive to stress.
The demonstration that the lumen of the ER contains several proteins at a relatively high concentration
raises important questions about the mechanism by
which they are precluded from proceeding along the
secretory pathway with secreted soluble proteins. Evidence exists that endoplasmin is even excluded from
the smooth ER (unpublished observation). The implication is that a barrier exists to prevent the diffusion of
this particular set of proteins within the ER, although
the nature of this sorting system is obscure. The only
clue is that all the known reticuloplasmins, endoplasmin, BIP, PDI and a 55xlO 3 M r protein possess the
same C-terminal sequence Lys-Asp-Glu-Leu (KDEL)
(Edman et al. 1985; Munro & Pelham, 1986; Sorger &
Pelham, 1987; Macer & Koch, unpublished), and it is
reasonable to expect that this sequence is important in
their specific retention by the ER. Clarification of the
mechanism involved in this sorting could provide
important clues about sorting mechanisms generally.
It is clear even from this brief article that our
understanding of the reticuloplasm and its contents is
still at a very primitive stage. What has emerged is that
most of the major ER proteins are actually localized in
the lumen and it becomes difficult to avoid the conclusion that many of the answers to the outstanding
questions relating to ER structure and function reside
in this area. Perhaps in the next few years there will
emerge a corresponding increase in a general awareness
of reticuloplasm and the reticuloplasmins in particular.
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