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Are there any examples in nature of two polypeptides join into a single, continuous, third polypeptide like this:

polypeptide-1 + polypeptide-2 = polypeptide-3

(Where all the indicated amino and carboxyl groups are on the main polypeptide chain ( located on Cα of the respective amino acids )

And if this exists, is there any term for the phenomenon?

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    $\begingroup$ This could be a type of en.wikipedia.org/wiki/Post-translational_modification - see the "Other proteins or peptides" section, although all of the examples given have covalent links to sidechains, not mainchains... $\endgroup$ – gilleain Feb 7 '17 at 13:15
  • $\begingroup$ @gilleain Yes you guessed correctly, I meant amino and carboxyl of alpha carbon. Okay I'm updating the question. $\endgroup$ – Always Confused Feb 7 '17 at 13:39
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I am not aware of anything precisely corresponding to your diagram, but the somewhat related behaviour of inteins may be of interest in this respect. They are defined in Wikipedia as:

An intein is a segment of a protein that is able to excise itself and join the remaining portions (the exteins) with a peptide bond in a process termed protein splicing.

So, unlike either ribosomal or non-ribosomal polypeptide (e.g. antibiotic) sythesis, they catalyse a peptide bond between peptides rather than an amino acid and a peptide. However these are already part of single polypeptide chain before the intein itself is spliced out.

The mechanism of protein splicing involving inteins.

[Modified from Intein_mech.png to fit page. — The mechanism of protein splicing involving inteins. In this scheme, the N-extein is shown in red, the intein in black, and the C-extein in blue. X represents either an oxygen or sulfur atom.]

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  • $\begingroup$ I see! This concept is also similar with transposon that occur in DNA. $\endgroup$ – Always Confused Feb 7 '17 at 14:02
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    $\begingroup$ @AlwaysConfused — Yes, and self-splicing introns. $\endgroup$ – David Feb 7 '17 at 14:19
  • $\begingroup$ hmm the wikipedia page includes the term "intron" so I didn't mentioned it. So Transposition:DNA::splicing:RNA::intein-excision:Protein :P $\endgroup$ – Always Confused Feb 7 '17 at 14:23
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    $\begingroup$ @AlwaysConfused — self-splicing, not splisosome splicing. Much rarer. $\endgroup$ – David Feb 7 '17 at 15:41
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    $\begingroup$ @AlwaysConfused — I should qualify my last remark. Self-splicing is similar to the intein phenomenon in that the material being cut and joined (RNA or protein) is catalysing the process. Transpons use proteins they encode. But of course you are right in saying that they are examples of cutting and joining pieces of DNA (as, indeed, are other phenomena involving DNA, like recombination). $\endgroup$ – David Feb 8 '17 at 14:07
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Does splicing of peptides in the proteasome count? Proteasomes normally degrade proteins into small peptides, but the process is conceptually reversible -- peptide bonds can be generated as well as broken -- which leads to splicing of two smaller peptides into one longer one. There is some evidence that this is a fairly common event:

Reports of proteasome-generated spliced epitopes exist, but they have been regarded as rare events. Here, however, we show that the proteasome-generated spliced peptide pool accounts for one-third of the entire HLA class I immunopeptidome in terms of diversity and one-fourth in terms of abundance.

--A large fraction of HLA class I ligands are proteasome-generated spliced peptides.

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