Table 1.

Split-inteins commonly used for PTS^a

	Sizeb		Half- life c (min)	Comments
	IntN	IntC
Naturally split inteins
Ssp DnaE	123	36	35–175	One of the better studied and broadly used split inteins. Requires three native extein residues (Cys-Phe-Asn) at the C-terminal junction. IntC is accessible to SPPSd.
Npu DnaE e	102	36	~1	Most efficient split intein described so far. Active with a broad set of residues at the splicing junction. IntC is accessible to SPPS.
Artificially split inteins
Mtu RecA	105	38	60–120	Reconstitution of the active intein requires co-refolding of previously denatured fragments.
Sce VMA	184	55	6	Requires induced fragment complementation by auxiliary dimerization domains. Has been used to control protein function in conditional protein splicing systems in vivo and in vitro.
Ssp DnaB-S0 f	104	47	12	Active under native conditions. Ser-Gly required at the N-terminal junction.
Ssp DnaB-S1 f	11	143	280	The short IntN is amenable to SPPS and has been used for the labeling of protein N-termini. Same sequence requirements than as DnaB-S0.
Ssp GyrB-S11 f	150	6	170	Active under native conditions. Ser-Ala-Asp used at the N-terminal junction.

^aSsp: Synechotcystis sp.; Npu: Nostoc punctiforme; Sce: Saccharomyces cerevisiae; Mtu: Mycobacterium tuberculosis. (Mootz, 2009) and references therein.

^bNumber of residues.

^cHalf-lives calculated from reported first order rate constants.

^dSolid-phase peptide synthesis.

^eArtificial variants of the Npu DnaE, with shorter Int^C (15 and 6 residues), have been designed by shifting the split site closer to the C-terminus.

^fS0, S1 and S11 indicate the site at which the intein is split. S0 corresponds to the split site of naturally split inteins.