Page 237 - Color Atlas of Biochemistry

228 Organelles

Protein sorting organelles (selective protein transfer). Struc-
tural signals can also activate enzymes that
modify the proteins and thereby determine
A. Protein sorting
their subsequent fate. Examples include lyso-
The biosynthesis of all proteins starts on free somal proteins (see p. 234) and membrane
ribosomes (top). However, the paths that the proteins with lipid anchors (see p. 214).
proteins follow soon diverge, depending on After they have been used, signal peptides
which target they are destined for. Proteins at the N terminus are cleaved off by specific
that carry a signal peptide for the ER (1) follow hydrolases (symbol: scissors). In proteins that
the secretory pathway (right). Proteins that do contain several successive signal sequences,
not have this signal follow the cytoplasmic this process can expose the subsequent sig-
pathway (left). nals. By contrast, signal peptides that have to
Secretory pathway. Ribosomes that syn- be read several times are not cleaved.
thesize a protein with a signal peptide for
the ER settle on the ER (see p. 228). The pep-
tide chain is transferred into the lumen of the C. Exocytosis
rER. Thepresenceorabsence of other signal Exocytosis is a term referring to processes
sequences and signal regions determines the that allow cells to expel substances (e. g., hor-
subsequent transport pathway. mones or neurotransmitters) quickly and in
Proteins that have stop-transfer sequences large quantities. Using a complex protein ma-
(4) remain as integral membrane proteins in chinery, secretory vesicles fuse completely or
the ER membrane. They then pass into other partially with the plasma membrane and re-
membranes via vesicular transport (see lease their contents. Exocytosis is usually
p. 226). From the rER, their pathway then regulated by chemical or electrical signals. As
leads to the Golgi apparatus and then on to an example, the mechanism by which neuro-
the plasma membrane. Proteins destined to transmitters are released from synapses (see
remain in the rER—e. g., enzymes—find their p. 348) is shown here, although only the most
way back from the Golgi apparatus to the rER important proteins are indicated.
with the help of a retention signal (2). Other The decisive element in exocytosis is the
proteins move from the Golgi apparatus to interaction between proteins known as
the lysosomes (3; see p. 234), to the cell SNAREs that are located on the vesicular
membrane (integral membrane proteins or membrane (v-SNAREs) and on the plasma
constitutive exocytosis), or are transported membrane (t-SNAREs). In the resting state
out of the cell (9; signal-regulated exocytosis) (1), the v-SNARE synaptobrevin is blocked by
by secretory vesicles (8). the vesicular protein synaptotagmin. When an
Cytoplasmic pathway. Proteins that do not action potential reaches the presynaptic
have a signal peptide for the ER are synthe- membrane, voltage-gated Ca 2+ channels
sizedinthe cytoplasm onfree ribosomes,and open (see p. 348). Ca 2+ flows in and triggers
remain in that compartment. Special signals the machinery by conformational changes in
mediate further transport into the mitochon- proteins. Contact takes place between synap-
dria (5; see p. 232), the nucleus (6; see p. 208) tobrevin and the t-SNARE synaptotaxin (2).
or peroxisomes (7). Additional proteins known as SNAPs bind to
the SNARE complex and allow fusion between
the vesicle and the plasma membrane (3). The
B. Translocation signals
process is supported by the hydrolysis of GTP
Signal peptides are short sections at the N or C by the auxiliary protein Rab.
terminus, or within the peptide chain. Areas Thetoxin of thebacterium Clostridium bot-
on the protein surface that are formed by ulinum, one of the most poisonous substances
varioussectionsof the chain orbyvarious there is, destroys components of the exocyto-
chains are known as signal regions.Signal sis machinery in synapses through enzymatic
peptides and signal regions are structural sig- hydrolysis, and in this way blocks neurotrans-
nals that are usually recognized by receptors mission.
on organelles (see A). They move the proteins,
with the help of additional proteins, into the

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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