Signal recognition particle binds to translating ribosomes before emergence of a signal anchor sequence

Abstract Co-translational protein targeting to membranes relies on the signal recognition particle (SRP) system consisting of a cytosolic ribonucleoprotein complex and its membrane-associated receptor. SRP recognizes N-terminal cleavable signals or signal anchor sequences, retards translation, and delivers ribosome-nascent chain complexes (RNCs) to vacant translocation channels in the target membrane. While our mechanistic understanding is well advanced for the small bacterial systems it lags behind for the large bacterial, archaeal and eukaryotic SRP variants including an Alu and an S domain. Here we describe recent advances on structural and functional insights in domain architecture, particle dynamics and interplay with RNCs and translocon and GTP-dependent regulation of co-translational protein targeting stimulated by SRP RNA.

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Interplay of signal recognition particle and trigger factor at L23 near the nascent chain exit site on the Escherichia coli ribosome

The Journal of Cell Biology ◽

10.1083/jcb.200302130 ◽

2003 ◽

Vol 161 (4) ◽

pp. 679-684 ◽

Cited By ~ 93

Author(s):

Ronald S. Ullers ◽

Edith N.G. Houben ◽

Amanda Raine ◽

Corinne M. ten Hagen-Jongman ◽

Måns Ehrenberg ◽

...

Keyword(s):

Escherichia Coli ◽

Ribosomal Proteins ◽

Signal Recognition Particle ◽

Attachment Site ◽

Trigger Factor ◽

Signal Recognition ◽

Exit Site ◽

Nascent Chain ◽

Signal Anchor

As newly synthesized polypeptides emerge from the ribosome, they interact with chaperones and targeting factors that assist in folding and targeting to the proper location in the cell. In Escherichia coli, the chaperone trigger factor (TF) binds to nascent polypeptides early in biosynthesis facilitated by its affinity for the ribosomal proteins L23 and L29 that are situated around the nascent chain exit site on the ribosome. The targeting factor signal recognition particle (SRP) interacts specifically with the signal anchor (SA) sequence in nascent inner membrane proteins (IMPs). Here, we have used photocross-linking to map interactions of the SA sequence in a short, in vitro–synthesized, nascent IMP. Both TF and SRP were found to interact with the SA with partially overlapping binding specificity. In addition, extensive contacts with L23 and L29 were detected. Both purified TF and SRP could be cross-linked to L23 on nontranslating ribosomes with a competitive advantage for SRP. The results suggest a role for L23 in the targeting of IMPs as an attachment site for TF and SRP that is close to the emerging nascent chain.

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Requirements for the membrane insertion of signal-anchor type proteins.

The Journal of Cell Biology ◽

10.1083/jcb.113.1.25 ◽

1991 ◽

Vol 113 (1) ◽

pp. 25-34 ◽

Cited By ~ 47

Author(s):

S High ◽

N Flint ◽

B Dobberstein

Keyword(s):

Signal Recognition Particle ◽

Membrane Insertion ◽

Type I ◽

Type Ii ◽

Nascent Chain ◽

Identical Manner ◽

Signal Anchor ◽

Protein Components ◽

Anchor Sequence ◽

Anchor Proteins

Proteins which are inserted and anchored in the membrane of the ER by an uncleaved signal-anchor sequence can assume two final orientations. Type I signal-anchor proteins translocate the NH2 terminus across the membrane while type II signal-anchor proteins translocate the COOH terminus. We investigated the requirements for cytosolic protein components and nucleotides for the membrane targeting and insertion of single-spanning type I signal-anchor proteins. Besides the ribosome, signal recognition particle (SRP), GTP, and rough microsomes (RMs) no other components were found to be required. The GTP analogue GMPPNP could substitute for GTP in supporting the membrane insertion of IMC-CAT. By using a photocrosslinking assay we show that for secreted, type I and type II signal-anchor proteins the presence of both GTP and RMs is required for the release of the nascent chain from the 54-kD subunit of SRP. For two of the proteins studied the release of the nascent chain from SRP54 was accompanied by a new interaction with components of the ER. We conclude that the GTP-dependent release of the nascent chain from SRP54 occurs in an identical manner for each of the proteins studied.

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