Cotranslational protein folding within the ribosome tunnel influences trigger-factor recruitment

Abstract

In recent years, various folding zones within the ribosome tunnel have been identified and explored through x-ray, cryo-electron microscopy (cryo-EM), and molecular biology studies. Here, we generated ribosome-bound nascent polypeptide complexes (RNCs) with different polyalanine (poly-A) inserts or signal peptides from membrane/secretory proteins to explore the influence of nascent chain compaction in the Escherichia coli ribosome tunnel on chaperone recruitment. By employing time-resolved fluorescence resonance energy transfer and immunoblotting, we were able to show that the poly-A inserts embedded in the passage tunnel can form a compacted structure (presumably helix) and reduce the recruitment of Trigger Factor (TF) when the helical motif is located in the region near the tunnel exit. Similar experiments on nascent chains containing signal sequences that may form compacted structural motifs within the ribosome tunnel and lure the signal recognition particle (SRP) to the ribosome, provided additional evidence that short, compacted nascent chains interfere with TF binding. These findings shed light on the possible controlling mechanism of nascent chains within the tunnel that leads to chaperone recruitment, as well as the function of L23, the ribosomal protein that serves as docking sites for both TF and SRP, in cotranslational protein targeting.

Figure 1

Experimental design of the TR-FRET experiments on poly-A-containing RNCs. (A) Schematic of RNCs in the presence and absence of poly-A segments within the ribosome tunnel. The poly-A fragments are shown in rectangles, with the fluorescent donor and acceptor indicated by red and blue circles, respectively. (B) Sequences of sG, 2A, 8A, and 17A. The parent sequences from GFP are in black and the poly-A inserts are in red. (C) Chemical structures of the fluorescent donor (BODIPY_FL) and acceptor (BODIPY₅₇₆) attached tRNAs. The fluorophores attached to Lys and Met are colored in green (donor) and red (acceptor), respectively. (D) Schematic of 2A with the poly-A insert in a totally helical (left) or extended (right) conformation. The correlation between E_FRET and the donor-acceptor separation distance (R) is described by Eq. 9.

Figure 2

PAGE analysis of fluorophore-labeled RNCs. (A) Fluorescence scanning of doubly labeled nascent chains and a blank experiment on SDS-PAGE with scanning of the donor (left panel) and acceptor (right panel) channels (see Materials and Methods). (B) Fluorescence image analysis of the donor-labeled RNCs with puromycin or RNase A treatment. By comparing sG and the free fluorophore-labeled tRNA (denoted C) on the PAGE, one can visualize the fluorescent bands of p-tRNA-sG, puromycin-sG, and fluorescent tRNA (BODIPY_FL-Lys-tRNA^Lys) and identify them by comparing the fluorescent signals before (−) and after (+) the addition of puromycin or RNase A. The smear band observed in the bottom is the hydrolyzed fluorophore during the electrophoresis.

Figure 3

Poly-A-containing nascent chains form a compacted conformation in the tunnel. (A) The lifetime measurements of the RNCs (sG, 2A, 8A, and 17A) were determined with the use of a frequency-domain fluorimeter. Data are shown for the donor-labeled samples (solid line) and double-labeled samples (dashed line). (B) The distance between the fluorophore pairs in different RNCs, derived from Eq. 9 and the measured E_FRET (Table 1) with R⁰ = 57 Å. (C) Representations of the structures of the nascent chains associated with the different RNCs. The poly-A fragments are shown as compacted helices in 2A, 8A, and 17A.

Figure 4

Poly-A and signal peptide containing RNCs decrease the recruitment of TF. (A) A schematic of the generation and characterization of the TF-RNCs. (B) Amino acid sequence alignment of the extended poly-A peptide (2A⁺, 8A⁺, and 17A⁺) and the signal peptide (SA and SS) on the RNCs. The parent amino acid sequences from GFP are in black and the inserted/substituted amino acids are in red. (C) Western blotting of the TF recruited by different RNCs (sG, 2A, 8A, 17A, 2A⁺, 8A⁺, 17A⁺, SA, and SS) with varying amounts of external TF (for details see Materials and Methods). (D) Ratios of the recruited TF relative to that recruited by sG with a 4 μM TF supplement. All experiments were repeated three to nine times, and the data are represented by means ± SD. Nascent chains that showed a significant difference (p smaller than 0.01) in TF recruitment from the sG control according to the t-test are highlighted by an asterisk (TF recruitments were performed with the same amount of TF supplement).

Figure 5

Possible interacting zones between the ribosome tunnel and the nascent chains suggested by the ribosome crystal structure (5,6) and our experimental results. (A) Possible locations of our nascent chains in the tunnel. The ribosome tunnel is colored in light blue, and the three ribosomal proteins that may interact directly with nascent chains in the tunnel are in yellow (L4), blue (L22), and red (L23) block, respectively. For easier comparison, the compact poly-A and signal sequence fragments on the nascent chains are shown as tubes and the parent GFP fragments are shown as dotted lines. The region of the ribosome tunnel 35 Å from the tunnel exit that includes the L23 is proposed to be important for TF recruitment. (B) A model of the compacted nascent chains (e.g., 8A) within the bottom of the tunnel that may interact with L23 and influence TF recruitment.