Ca. Reymillet et al., ASSOCIATED COMPLEMENT C3B - TOWARDS AN UNDERSTANDING OF ITS INTRACELLULAR MODIFICATIONS, Molecular immunology, 30(10), 1993, pp. 855-864
Covalent Superose microspheres-bound C3b was used as a model system to
simplify the analysis of antigen-bound C3b modifications during antig
en processing. The model was set up using purified C3 and Superose-bou
nd trypsin. C3b was covalently bound to Superose through an ester link
, as indicated by lability to hydroxylamine treatment at alkaline pH.
C3b-Superose was incubated with L subcellular fraction, enriched in en
dosomes/lysosomes, purified from U937 cell line. Two types of limited
activities on the C3b-Superose model system were detected: (i) a prote
olytic activity cleaving C3b into mainly a C3c-like fragment which was
released and a C3d-like fragment of apparent M(r) 32 kDa which remain
ed bound to Superose through the original ester link; (ii) an esteroly
tic activity cleaving the ester bond and releasing C3b. Inhibition exp
eriments pointed to the involvement of serine, aspartyl and cysteine p
roteases. Cathepsin B appeared most probably as one of the major prote
ases of L fraction catalysing the proteolysis of the C3b-bound. Kineti
c studies were in favour of a good stability of the ester bond, suppor
ting an effective role of C3b as a chaperone during the extracellular
and intracellular travel of C3b-bound antigen.