Protein Splicing of Inteins and Hedgehog Autoproteolysis: Structure, Function, and Evolution

The Mxe GyrA mini-intein represents a splicing element without a DRR or core endonuclease domain. Larger inteins, like the Sce VMA intein, contain homing endonucleases. The structural similarity among splicing elements and Hh-C17 can be seen in the common positioning of β strands and the large number of residues with superimposable Cα atoms (the main chain carbon atom to which the side chain is attached) (Figure 3Figure 3). However, there is little sequence identity among the three proteins except for the residues involved in catalysis (Figure 1Figure 1 and Figure 3Figure 3). Because of the similarity of their core structures, Hall et al. 1997xHall, T.M.T, Porter, J.A, Young, K.E, Koonin, E.V, Beachy, P.A, and Leahy, D.J. Cell. 1997; 91: 85–97Abstract | Full Text | Full Text PDF | PubMed | Scopus (150)See all ReferencesHall et al. 1997 proposed that inteins and Hh-C17 evolved from a common precursor (Figure 4Figure 4). The Hint module mediates ester/thioester formation, activating the linkage between the element and a second protein domain. Other systems, such as the Ntn hydrolase family, employ an acyl rearrangement to activate catalysis but do not have structural similarity to the Hint module (Hall et al. 1997xHall, T.M.T, Porter, J.A, Young, K.E, Koonin, E.V, Beachy, P.A, and Leahy, D.J. Cell. 1997; 91: 85–97Abstract | Full Text | Full Text PDF | PubMed | Scopus (150)See all ReferencesHall et al. 1997). Inteins and Hh-C subsequently evolved separate methods of cleaving this bond. Inteins evolved the ability to ligate two exteins and acquired the DRR and core endonuclease. The endonuclease allowed inteins to spread by lateral transmission. Hh-C17 acquired a sterol recognition region (SRR) directing addition of cholesterol to the hedgehog protein signaling domain, anchoring the signaling domain to the cell surface. Alternatively, the Hint module could have invaded a preexisting signaling domain/SRR element. Several nematode Hh-C domains contain unrelated C-terminal extensions that may interact with molecules other than cholesterol and have been tentatively termed adduct recognition regions (Beachy et al. 1997xCold Spring Harbor Symp. Beachy, P.A, Cooper, M.K, Young, K.E, von Kessler, D.P, Park, W, Hall, T.M.T, Leahy, D.J, and Porter, J.A. Quant. Biol. 1997; in pressSee all ReferencesBeachy et al. 1997). The order of these events is speculative, including the order of module assembly. Each of the intein and hedgehog elements may have coevolved or associated after independent formation. Endonucleases may have also been lost from inteins. As we understand the mechanism of protein splicing and Hh-C autoproteolysis, we will begin to be able to harness these elements to cleave or splice any target protein at will.Figure 4A Scenario for Evolution of Inteins and Hh-CA primordial subdomain was duplicated and loop exchange occurred between subdomains to generate the Hint module. Inteins then acquired the DNA recognition region (DRR) and core endonuclease domain (ENDO) and Hh-C17 acquired a sterol recognition region (SRR). The order of these events is speculative. Adapted with permission from Hall et al., 1997.View Large Image | View Hi-Res Image | Download PowerPoint Slide