Protein splicing is a post-translational
process by which an intervening
protein, or an intein, catalyzes its own excision from flanking polypeptides,
or exteins, coupled to extein ligation. Four inteins interrupt the
MCM helicase of the halophile Haloquadratum walsbyi, two of which are mini-inteins that lack a homing endonuclease.
Both inteins can be overexpressed in Escherichia coli and purified as unspliced precursors; splicing can be induced in vitro by incubation with salt. However, one intein can
splice in 0.5 M NaCl in vitro, whereas the other
splices efficiently only in buffer containing over 2 M NaCl; the organism
also requires high salt to grow, with the standard growth media containing
over 3 M NaCl and about 0.75 M magnesium salts. Consistent with this
difference in salt-dependent activity, an intein-containing precursor
protein with both inteins promotes conditional alternative protein
splicing (CAPS) to yield different spliced products dependent on the
salt concentration. Native Trp fluorescence of the inteins suggests
that the difference in activity may be due to partial unfolding of
the inteins at lower salt concentrations. This differential salt sensitivity
of intein activity may provide a useful mechanism for halophiles to
respond to environmental changes.