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Post
Translational
Modification -
Primary
translation
products
undergo
a
whole
variety
of
modifications,
which
are
vital
to
the
production
of
a
fully
functional
protein.
These
post
translational
protein
processing
steps
contribute
to
post
translational
modifications.
These
steps
are
more
varied
and
more
important
in
eukaryotes.
One
reason
for
this
is
the
existence
of
more
membrane
bound
compartments
into
which
proteins
must
be
directed.
Polysomes
can
occur
either
free
in
the
cytoplasm
or
bound
to
membranes
forming
rough
endoplasmic
reticulum.
The
membrane
bound
polysomes
are
synthesizing
proteins
which
are
either
secreted
from
the
cell,
form
part
of
a
membrane
or
are
packaged
into
specialized
organelles
(e.g.
seed
storage
protein
bodies).
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How
is
polysome
attachment
to
membranes
mediated,
and
how
does
this
correlate
with
the
transport
of
proteins
to
their
final
cellular
locations?
It
has
been
proposed
that
the
polypeptide
itself
carries
a
signal
at
or
near
the
N-terminus
which
mediates
attachment
to
the
membrane.
Translocated
proteins
carry
an
N-terminal
extension
of
about
twenty
amino
acids,
termed
a
signal
peptide;
it
binds
to
a
receptor
in
the
membrane
as
soon
as
it
is
synthesized
and
emerges
from
the
ribosome. "Binding
initiates
transport
across
the
membrane,
which
occurs
concurrent
with
translation,
and
is
accompanied
by
removal
of
the
signal
peptide
by
a
specific
membrane-associated
peptidase.
Analysis
of
signal
peptides
from
different
polypeptides
has
shown
that
there
is
no
strict
sequence
conservation
between
them.
However,
the
central
portion
of
each
signal
peptide
is
hydrophobic
and
a
number
of
methionine
residues
often
tend
to
be
distributed
along
the
length.
Signal
peptides
have
been
found
on
the
primary
translation
products
of
a
number
of
plant
mRNAs;
e.g.,
on
zein
from
maize.
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In
addition
to
the
removal
of
signal
peptides
from
proteins,
other
modifications
are
carried
out.
Glycosylation
occurs
during
passage
of
proteins
through
a
membrane,
via
dolicol
phosphate
intermediates;
the
protein
specifies
its
own
glycosylation
sites
which
occur
at
residues
ASN-X-SER,
where
X
is
any
amino
acid
and
the
glycoside
unit
is
added
in
the
serine
residue.
Many
plant
proteins
are
glycosylated
in
this
way.
Many
proteins
remain
inactive
until
their
polypeptide
chains
are
cleaved
by
proteases.
This
allows
their
enzymatic
activity
to
be
controlled
with
regard
to
time
and
space.
It
also
allows
their
enzymatic
activity
to
be
controlled
by
changes
in
environment.
Examples
of
such
proteins
include
insulin,
which
has
two
chains
cleaved
from
a
large
single
chain
called
proinsulin.
(Proinsulin
in
turn
comes
from
preproinsulin
through
the
removal
of
the
latter's
leader
sequence.)
Other
examples
include
trypsin
and
chymotryspin.
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An
even
more
spectacular
example
of
protein
processing
in
eukaryotes
involves
the
synthesis
of
multiple
gene
products
from
one
gene.
The
original
gene
product
is
a
single
polypeptide
chain.
The
functional
molecules
are
fragments
of
this
chain
produced
by
proteolytic
cleavage.
Sometimes
different
types
of
functional
proteins
are
produced
by
cleavage
at
different
points
in
the
chain.
The
best
studied
example
is
the
complex
of
polypeptide
hormones
produced
by
the
pituitary
gland.
Other
types
of
processing
involve
the
addition
of
moieties
to
the
amino
acid
residues.
In
addition
to
the
carbohydrates
added
in
the
Golgi
apparatus,
moieties
such
as
lipids,
complex
cofactors,
and
phosphate
groups
may
be
added
to
proteins.
Phosphorylation,
in
which
a
phosphate
is
transferred
from
ATP
to
a
serine,
threonine,
or
tyrosine
residue
by
a
protein'
kinase,
is
especially
important,
since
it
forms
one
or
more
steps
in "cascades" of
events
controlling
important
biochemical
pathways.
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Phosphorylation
plays
a
part
in
such
biochemical
processes
as
glycogen
metabolism,
glycolysis,
gluconeogenesis,
and
protein
synthesis.
In
glycogen
metabolism
for
instance,
the
activity
of
the
enzyme
glycogen
phosphorylase
depends
on
whether
a
key
serine
residue
has
been
phosphorylated
by
phosphorylase
kinase
or
whether
it
has
been
dephosphorylated
by
a
protein
phosphatase.
The
activity
of
phosphorylase
kinase
is
itself
dependent
on
phosphorylation
and
thus
on
the
relative
activities
of
a
protein
kinase
and
protein
phosphatases
In
conclusion,
it
can
be
seen
that
in
eukaryotes
there
are
many
steps,
each
of
which
must
function
properly
for
the
successful
expression
of
a
gene.
Thus
there
are
many
potential
points
at
which
gene
expression
may
be
controlled.
There
is
also
evidence
that
there
are
many
points
in
eukaryotes
at
which
gene
expression
is
controlled.
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Many
examples
of
gene
activation
can
be
found,
including
special
cases
involving
immunoglobulin
gene
rearrangements
and
amplification
of
rDNA.
And
there
is
evidence
for
differential
control
of
transcription
(though
this
may
be
indistinguishable
from
gene
activation).
Indirect
evidence
exists
for
control
at
the
level
of
hnRNA.
Control
at
the
level
of
translation
occurs
in
reticulocytes
that
are
deficient
in
heme:
Phosphorylation
of
an
elongation
factor
stops
translation
of
the
globin
mRNA.
Post
translational
processing,
involving
proteolysis
and
other
processes,
controls
the
activity
of
numerous
enzymes
and
the
final
steps
in
the
synthesis
of
some
vertebrate
peptide
hormones.
The
foregoing
list
of
examples
should
not
obscure
the
fact
that
we
know
very
little
about
the
control
of
most
genes.
We
certainly
have
no
general
principles
on
which
to
base
predictions.
For
many
situations
in
plant
development,
we
are
still
in
the
initial
stage,
trying
to
decide
what
genes
to
study
and
at
what
level
the
control
of
gene
expression
occurs.
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