Biology Reference
In-Depth Information
The [PSI
+
], [PIN], [URE3], and
[Het-s] Phenotypes
6.3
The [PSI
+
] and [PIN] Traits
6.3.1
+
The [PSI
] trait is associated with Sup35p, an essential component
of the translation termination machinery.
10,11
Indeed, the eukaryotic
release factor 3 (eRF3) or Sup35p, constitutes together with eRF1
or Sup45p, the functional translation release factor that recognizes
stop codons and releases nascent polypeptides from the final
t-RNA.
12-14
−
] cells, while it
is in functionally compromised aggregated form in [PSI
Sup35p is soluble and functional in [psi
+
15,16
] cells.
−
Thus, while translation termination is efficient in [psi
] cells, it is
+
not in [PSI
] cells, and the ribosomes read-through stop codons and
synthesize abnormal polypeptides.
Sup35p (Swiss-Prot P05453) is a large polypeptide made of 685
amino acid residues with a calculated molecular mass of 76.5 kDa.
17
Three structural/functional regions have been distinguished within
Sup35p (Fig. 6.1). The N-terminal region (N) that extends from
amino acid residues 1 to 122 is rich in Q, N, and G residues (47%).
This region is not essential for the role of the protein in translation
termination.
18-20
It is called prion domain (PrD) as it plays a critical
role in prion propagation. The C-terminal region is the functional
domain of the protein providing translation-termination activity.
13,18
The latter domain is compactly folded based on the crystal structure
of eRF3 from
21
and contains the GTP and eRF1-binding
sites (Fig. 6.1; PDB accession code 1R5N). Finally, the middle region
(M) extending from amino acid residues 123 to 253 links the PrD
and the functional domain.
S. pombe
]
appearance. This is also what is observed upon the overexpression of
Rnq1p (Swiss-Prot P25367), a 42.5 kDa polypeptide, with unknown
function. Rnq1p is rich in Q and N residues particularly in its C-term-
inal part extending from amino acid residue 153 to 405 (Fig. 6.1).
Rnq1p is probably a natively unfolded polypeptide. It exists as a
soluble protein as well as in insoluble aggregates.
The overexpression of Sup35p increases the frequency of [PSI
+
22
The amount of
insoluble Rnq1p increases when the protein is overproduced.
23
In
−
its insoluble state, Rnq1p allows the conversion of [psi
] cells into
+
+
[PSI
] cells and is at the origin of the phenotype [PIN
] that comes
from “[PSI
+
] inducibility”.
5
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