Download Regulatory roles for the ribosome in protein targeting to the

A20
Biochemical Society Transactions (200 I) Volume 30, Part I
F12 Assembly and function of protein-RNA sub-complexes of the
GI
mammalian signal recognition particle
S.Cusa&, 0.Weichenrieder, K. Wild, I. Sinning
and K. Strub
EMBL Grenoble Outstation, BP181,38042 Grenoble Cedex 9,
France
dementia
D.A. Lomas,A.Lourbakos, S-A.Cumming and D.Belorgey
Respiratory Medicine Unit, Department of Medicine, University of
Cambridge, Cambridge Institute for Medical Research, Wellcome
Trust/MRC Building, Hills Road, Cambridge, CB2 2 X l : UK
The signal recognition particle (SRP) is a conserved ribonucleoproteia complex that mediates co-translational targeting of signal
peptide bearing secretory and membrane proteins to cellular
membranes. The mammalian SRP comprises the 300 nucleotide 7SL
SRP RNA complexed with six proteins. The SRP14/9 heterodimer
binds to the 3’ and 5’ extremities of SRP RNA forming the so-called
Ah-domain, which functions to retard or arrest translation while
the docking process occurs. The SRP68/72 heterodimer as well as
SRP19 and SRP54 bind to the central sequences of the RNA and
constitute the S-domain of SRP. SRP19 is an assembly protein which
must bind to S-domain RNA before SRP54. SRP54, a GTPase,
binds the RNA as well as the signal peptide and also interacts with
the SRP receptor. The crystallographic structure of the Ah-domain
of human SRP will be presented and its implications for the
assembly of this sub-domain and its function in elongation arrest
will be discussed. A second structure, of SRP19 bound to its
primary binding site on SRP RNA with includes a conserved
GNAR tetraloop, will also be presented. This structure gives insight
into the folding of the S-domain and the requirement for prior
binding of SRP19 before that of SRP54.
Alpha-1-antitrypsin functions as a mousetrap to inhibit its target
proteinase neutrophil elastase. The common severe Z deficiency
variant (Glu342Lys) destabilises the mousetrap to allow a sequential
interaction between the reactive centre loop of one molecule and Psheet A of another. These reactive loop-P-sheet A polymers
accumulate within hepatocytes to form inclusion bodies that are
associated with neonatal hepatitis, juvenile cirrhosis and hepatocelM a r carcinoma. Loop-sheet polymerisation also underlies the
plasma deficiency seen with mutants of other members of the serine
proteinase inhibitor (serpin) superfamily. In particular deficiency
mutants of antithrombin, C1-inhibitor and al-antichymotrypsin
have been demonstrated to form inactive polymers in association
with thrombosis, angio-oedema and chronic obstructive pulmonary
disease respectively. Moreover we have recently shown that the same
process in a neurone specific protein, neuroserpin, underlies a novel
inclusion body dementia. Our understanding of the structural basis
of polymerisation has allowed the development of strategies to
prevent the aberrant protein-protein interaction in vitro. This must
now be achieved in vivo if we are to treat the associated clinical
syndromes.
F13 Regulatory roles for the ribosome in protein targeting to the
G2
endoplasmic reticulum
M.Pool, T. Fulgal, I. Sinning1, B. Dobberstein
ZMBH, INF 282, 69111, Heidelberg, Germany, ‘BZH, INF 328,
69120, Heidelberg
Proteins destined for secretion are synthesised with an N-terminal
signal sequence which targets them to the endoplasmic reticulum
(ER) membrane. Targeting occurs co-translationally and is catalysed
by the signal recognition particle (SRP), together with the SRP
receptor (SR), a heterodimeric intergral ER membrane protein. The
targeting reaction is controlled by the action of three GTPases:
SRP54 and the a and P subunits of SR. We have identified the
ribosome as a regulator of two of these GTPases.
In the first step of targeting, a ribosomal component, probably L35,
activates SRP54 by increasing its affinity for GTP. The ribosomeSRP complex is then targeted to the ER by the interaction of SRP54
with SRa, inducing high affinity GTP binding to both proteins.
Release of the signal sequence from SRP requires the presence of the
translocon. This is sensed via the third GTPase SRP. In the absence
of the translocon, SRP is stabilized in the nucleotide-free state by a
21 kDa ribosomal protein. Binding of the ribosome to the
translocon destabilizes this interaction, allowing GTP to enter SRP
which in turn triggers signal sequence release. The ribosome thereby
regulates SRP, such that signal sequence release is coordinated with
the presence of the translocon.
0 2002 Biochemical Society
Hypersensitive mouse traps, al-antitrypsin deficiency and
Gene regulation of the serine proteinase inhibitors
a 1-antitrypsin and a 1-antichymotrypsin
N.A.Kalsheker,K. Morgan,S.Morley
Division of Clinical Cbemistry,Scbool of Clinical Laboratory
Sciences, University Hospital, Queens Medical Centre, Nottingham
NG7 2UH
The serine proteinase inhibitors (serpins) are a superfamily of
proteins with a diverse set of functions including the control of
blood coagulation, complement activation, programmed cell death
and development.The most abundant serpins in human plasma are
al-antitrypsin (AAT) and al-antichymotrypsin (ACT). During
inflammation, circulating levels can increase by up to three-fold for
AAT and five-fold for ACT. The major site for increased synthesis
is the liver. Other tissues, such as the lung, are also capable of
synthesis and expression can be increased up to 100-fold by
cytokines.
For AAT there is a tissue-specific promoter for the liver and an
alternative promoter for other tissues. The tissue-specific transcription factors H N F - l a and HNF-4 acting synergistically regulate
basal expression. An enhancer in the 3’ flanking sequence modulates
cytokine induced expression by interleukin-6 and oncostatin-M.
There is probably a single promoter for ACT. Microcell hybrid
transfection studies have shown that a sequence containing about 15
kb of 5’ flanking sequence is sufficient to allow stable expression of
AAT in a position-independent manner. These studies should
facilitate fine mapping of the elements required for expression.