Correlation between covalent attachment of C3 and calcium uptake in antibody-stimulated L cells

The nature of C3 binding to cell surfaces was examined in L cells treated with antibody and complement, immunological reagents which we have previously shown to be capable of producing stimulation of several important cellular processes. In the presence of serums containing C3 and under experimental conditions where complement activation could take place, selective binding of C3 to antibody treated L cells was observed (maximum 1.1 X 10(6) C3 molecules per cell). Under similar conditions there was a C3 dependent increased calcium uptake (3.4 pmol) by antibody treated cells. Purified C3 was able to selectively restore C3 binding to cells treated with serum depleted of C3 through C9. C7-deficient serum was almost as good a source of activated C3 as its normal serum counterpart. Strong chemical nucleophiles such as salicylhydroxamic acid, which are capable of covalently coupling to the labile internal thiolester of C3, prevented the binding of C3 to cells. We conclude that C3 is covalently bound to antibody and complement treated L cells, possibly serving as an important signal in subsequent enhancement of phospholipid metabolism, DNA synthesis, and cell growth.