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628s Biochemical SocietyTransactions( 1 996) 24
6 1 FUNCTION OF THE PROTEASOME IN PROTEIN
TURNOVER AND ANTIGEN PRESENTATION.
Alfred L. G o l d b u . Dept. of Cell Biology, Harvard
Medical School, Boston, MA 02115, U.S.A.
63 INTRACELLULAR PROTEIN
The proteasome is the primary site in cells for the
complete degradation of cell proteins and for
production of most antigenic peptides presented to the
immune system on MHC-class I molecules. In this
process, intracellular proteins are degraded to 8-9
residue fragments, are then transported into the ER,
and become associated with class I molecules. 20s and
2 6 s proteasomes degrade proteins processively to
oligopeptides of 7-11 residues in length. Selective
inhibitors of proteasome function (peptide aldehyde or
lactacystin) inhibit the degradation of short-lived and
long-lived cell proteins and prevent the appearance of
antigenic peptides on the cell surface. Mutations that
block or promote ubiquitin conjugation prevent or
stimulate antigen presentation. y-Interferon (y-IFN)
induces new proteasome subunits, LMP2 and LMP7.
Their incorporation in place of constitutive subunits
increases cleavage of model peptides after basic and
hydrophobic residues, and decreases after acidic
residues. Transfections with LMP2 or LMP7 genes
cause similar changes in peptidase activities. These
changes should enhance production of peptides
preferentially transported into the ER and bound to
class I molecules and enhance immune surveillance.
Brian F.C. Clark & Suresh I.S. Rattan
Laboratory of Cellular Ageing
Institute of Molecular and Structural Biology
Aarhus University, 8000 Aarhus C, Denmark
62 BACTERIAL TOXINS AND CELL
WEWBRANETRAFFIC
Cesare MONTECUCCO
Dipartimento d i Scienze Biomediche, U n i v e r s i t a
d i Padova, V i a Trieste 75, PADOVA, ITALY
C e l l membrane t r a f f i c i s a h i g h l y c o o r d i n a t e d
phenomenon e s s e n t i a l for t h e d e l i v e r y and
uptake of p r o t e i n s ,
l i p i d s and smaller
molecules. B a c t e r i a produce a v a r i e t y of
protein toxins t h a t interfere w i t h v i t a l
a s p e c t s of c e l l physiology. It i s now emerging
t h a t bacteria produce t o x i n s t h a t e n t e r s i n t o
c e l l s and a f f e c t a selected s t e p of c e l l
membrane t r a f f i c k i n g . Two examples w i l l be
provided h e r e .
1. Some C l o s t r i d i a produce n e u r o t o x i n s , which
are m e t a l l o p r o t e i n a s e s able to e n t e r i n t h e
neuron c y t o s o l and r e c o g n i z e s p e c i f i c a l l y t h e
t h r e e SNARE p r o t e i n s . These f i n d i n g s i d e n t i f y
VAMP,
SNAP-25
and s y n t a x i n as e s s e n t i a l
components of t h e e x o c y t o s i s machinery and
i n d i c a t e t h a t n e u r o t o x i n s are new t o o l s i n t h e
s t u d y of e x o c y t o s i s . Examples of t h e u s e of
t h e s e t o x i n s i n c e l l b i o l o g y and i n medicine
w i l l be p r e s e n t e d .
2 . Toxigenic s t r a i n s of Helicobacter p y l o r i
cause
gastrointestinal
ulcers
and
are
a s s o c i a t e d to stomach adenocarcinomas. They
produce vacA, a c y t o t o x i n t h a t induces t h e
formation and growth of c e l l u l a r vacuoles t h a t
f i l l t h e c e l l volume and t h e c e l l d i e s . W e
have found t h a t t h e v a c u o l a r ATPase proton
pump i s p r e s e n t on v a c u o l e s a n d . a c i d i f i e s
t h e i r lumen. Evidence w i l l be p r e s e n t e d t h a t
vacuoles o r i g i n a t e f r o m a rab7 c o n t r o l l e d
homotypic
fusion
of
late
endosomal
compartments, which t h e n s w e l l f o l l o w i n g t h e
accumulation of ammonium i o n s , produced by
h y d r o l y s i s of u r e a by t h e powerful H . p y l o r i
urease.
SYNTHESIS, POST-TRANSLATIONAL
MODIFICATIONS AND AGEING
Changes in the functioning of proteins during
ageing can be due to inefficient and inaccurate
protein synthesis, altered pattern of post-translational modifications, and defective pathways of
protein turnover. Slowing-down of bulk protein
synthesis is a widely recognized biochemical
change with age. Elongation factors appear to play
a crucial role in the regulation of protein synthesis
during ageing. One of the reasons for the accumulation of abnormal proteins during ageing is
post-synthetic modifications including oxidation of
amino acids, deamidation, racemization, and spontaneous changes in protein conformation. The
relation between protein synthesis, modifications
and turnover, and ageing remains to be elucidated.