Serine proteases target invasin. (a) Wild-type Y. enterocolitica (wt) or the invasin mutant (inv) were digested with small intestinal fluid from mice (SI), α-chymotrypsin (CT), trypsin (T), carboxypeptidase A (CP), pepsin (P), elastase (E) or trypsin digestion buffer (control) for 2 h at 37°C. SI was washed with 2 ml PBS, centrifuged, and supernatant was used for digests. 1 × 107 CFU of yersiniae were subjected to Western blotting with an a inv polyclonal AB. (b) Y. enterocolitica WA-314 (wt) or the invasin mutant (inv) were digested with the supernatant of small intestinal luminal fluid (SI), mock digested with PBS or digested with SI preincubated with a1-antitrypsin for 1 h at 37°C. 1 × 107 CFU of yersiniae were subjected to Western blotting with an α inv polyclonal AB. Solid arrowheads point to full length Inv, open arrowheads point to specific Inv degradation products.