Antibody-independent interaction between the first component of human complement, C1, and the outer membrane of Escherichia coli D31 m4.

The heptoseless mutant of Escherichia coli, E. coli D31 m4, binds C1q and C1 at 0 degrees C and at low ionic strength (I0.07). Under these conditions, the maximum C1q binding averages 3.0 X 10(5) molecules per bacterium, with a Ka of 1.4 X 10(8) M-1. Binding involves the collagen-like region of C1q, as shown by the capacity of C1q pepsin-digest fragments to bind to E. coli D31 m4, and to compete with native C1q. Proenzyme and activated forms of C1 subcomponents C1r and C1s and their Ca2+-dependent association (C1r-C1s)2 do not bind to E. coli D31 m4. In contrast, the C1 complex binds very effectively, with an average fixation of 3.5 X 10(5) molecules per bacterium, and a Ka of 0.25 X 10(8) M-1, both comparable with the values obtained for C1q binding. C1 bound to E. coli D31 m4 undergoes rapid activation at 0 degrees C. The activation process is not affected by C1-inhibitor, and only slightly inhibited by p-nitrophenyl p'-guanidinobenzoate. No turnover of the (C1r-C1s)2 subunit is observed. Once activated, C1 is only partially dissociated by C1-inhibitor. Our observations are in favour of a strong association between C1 and the outer membrane of E. coli D31 m4, involving mainly the collagen-like moiety of C1.