Stop transfer signal

Fig. 3. Classification of single-spanning membrane proteins based on topology, (a) The loop model for explaining the biogenesis of type I topology in the translocon. The stop-transfer signal stops the integration, (b) Type I protein and a cleaved signal peptide, (c) Type II (NcytCexo) is made by a type II signal-anchor, (d) Type III (Nexo-Ccyto often called type I) is made by a type I signal-anchor, (e) Type IV (C-tail) is made independently from the translocon.

The inner envelope membrane proteins have a cleavable N-terminal transit peptide, as well as some hydrophobic domain (s) in their mature portion. There are two possibilities on the role of this hydrophobic domain it may work as an N-terminal signal peptide after the translocation into the stroma and the subsequent cleavage of the transit peptide. Alternatively, it may work as a stop-transfer signal. One more important question is how the distinction is made between the outer membrane proteins, the inner membrane proteins, and the thylakoid membrane proteins. It is still an enigma. 

If the growing polypeptide contains a stop-transfer signal (see p. 228), then this hydro-phobic section of the chain remains stuck in the membrane outside the translocon, and an integral membrane protein arises. In the course of translation, an additional signal sequence can re-start the transfer of the chain through the translocon. Several repetitions of this process produce integral membrane proteins with several transmembrane helices (see p. 214). 

Topogenic sequences (eg, signal [amino terminal or internal] and stop-transfer) are important in determining the insertion and disposition of proteins in membranes. 

At one end of the loop of tRNA there is a ribonucleotide triplet called anticodon which is complementary to a codon on mRNA. Each codon of mRNA is read in a serial order by an anticodon of tRNA and matched. If matching occurs, the tRNA transfers the desired amino acid to the growing polypeptide chain on the ribosome. When the synthesis of a specific protein is completed, a stop codon signals the end and the synthesized protein is released from the ribosome. 

This a-helix is amphipathic, containing patches of positively charged and hydrophobic amino acids, respectively, on opposite surfaces of the theoretical cylinder. The presequence is usually processed by the mitochondrial processing peptidase (MPP) and the mature protein is sorted to either the matrix, or to the inner membrane if it bears a hydrophobic stop-transfer sequence. Some mitochondrial proteins, mostly destined to the membranes, do not have cleavable N-terminal presequences but have internal targeting signals that are not well characterized (Pfanner and Geissler 2001). 

Fig. 4. Insertion of integral membrane proteins into the ER membrane during synthesis, (a) Type I integral membrane protein with a cleavable N-terminal signal sequence and a stop-transfer sequence (b) Type II integral membrane protein with an uncleaved N-terminal signal sequence (c) Type III integral membrane protein with multiple signal and stop-transfer sequences.

As each succ e9ive codan nn mRNA i it ad, dilTercnt tRNA s brin t the correct amino acids into position for en rme mediated transfer to the grow ing peptide. When synthesis of the proper pfvu in in completed, u stop codon signals the end. and the protein is released from the nbosome. TiU process is illustrated schematically in Figure 28.11. 

Internal Stop-Transfer and Signal-Anchor Sequences Determine Topology of Single-Pass Proteins... 

Initiates cotranslational translocation of the protein through the combined action of the SRP and SRP receptor. Once the N-terminus of the growing polypeptide enters the lumen of the ER, the signal sequence is cleaved, and the growing chain continues to be extruded across the ER membrane. However, unlike the case with secretory proteins, a sequence of about 22 hydrophobic amino acids in the middle of a type I protein stops transfer of the nascent chain through the translo-con (Figure 16-11). This Internal sequence, because of Its hydrophobicity, can move laterally between the protein subunits that form the wall of the translocon and become anchored in the phospholipid bilayer of the membrane, where It remains. Because of its dual function, this sequence Is called a stop-transfer anchor sequence. 

A FIGURE 16-11 Synthesis and insertion into the ER membrane of type I single-pass proteins. Step After the ribosome/nascent chain complex becomes associated with a translocon in the ER membrane, the N-terminal signal sequence is cleaved. This process occurs by the same mechanism as the one for soluble secretory proteins (see Figure 16-6). StepsH.H The chain is elongated until the hydrophobic stop-transfer anchor sequence is synthesized and enters the translocon, where it prevents the nascent chain from extruding farther into the ER... 

After synthesis of the first two transmembrane a helices, both ends of the nascent chain face the cytosol and the loop between them extends into the ER lumen. The C-terminus of the nascent chain then continues to grow into the cytosol, as it does in synthesis of type 1 and type 111 proteins. According to this mechanism, the third a helix acts as another type 11 signal-anchor sequence, and the fourth as another stop-transfer anchor sequence (see Figure 16-13d). Apparently, once the first topogenic sequence of a multipass polypeptide initiates association with the translocon, the ribosome remains attached to the translocon, and topogenic sequences that subsequently emerge from the ribosome are threaded into the translocon without the need for the SRP and the SRP receptor. 

STA= Internal stop-transfer anchor sequence SA-II = Internal signal-anchor sequence SA-III = Internal signal-anchor sequence... 

Other than its hydrophobicity, the specific amino acid sequence of a particular helix has little bearing on its function. Thus the first N-termlnal a helix and the subsequent odd-numbered ones function as signal-anchor sequences, whereas the intervening even-numbered helices function as stop-transfer anchor sequences. 

Some cell-surface proteins are anchored to the phospholipid bllayer not by a sequence of hydrophobic amino acids but by a covalently attached amphipathic molecule, gfyco sylphosphatidylinositol (GPI) (Figure 16-14a). These proteins are synthesized and initially anchored to the ER membrane exactly like type I transmembrane proteins, with a cleaved N-terminal signal sequence and Internal stop-transfer anchor sequence directing the process (see Figure... 

Figure 16-15 shows the hydropathy profiles for three different membrane proteins. The prominent peaks in such plots Identify probable topogenic sequences, as well as their position and approximate length. For example, the hydropathy profile of the human growth hormone receptor reveals the presence of both a hydrophobic signal sequence at the extreme N-termlnus of the protein and an internal hydrophobic stop-transfer sequence (see Figure 16-15a). On the basis...

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