A transglutaminase immunologically related to tissue transglutaminase catalyzes cross-linking of cell wall proteins in Chlamydomonas reinhardtii

The addition of primary amines to the growth medium of the unicellular gree n alga Chlamydomonas reinhardtii disrupts cell wall assembly in both vegeta tive and zygotic cells. Primary amines are competitive inhibitors of the pr otein-cross-linking activity of transglutaminases. Two independent assays f or transglutaminase confirmed a burst of extracellular activity during the early stages of cell wall formation in both vegetative cells and zygotes. W hen noninhibiting levels of a radioactive primary amine (C-14-putrescine) w ere added to the growth medium, both cell types were labeled in a reaction catalyzed by extracellular transglutaminase. The radioactive label was foun d specifically in the cell wall proteins of both cell types, and acid hydro lysis of the labeled material released unmodified C-14-putrescine. Western blots of the proteins secreted at the times of maximal transglutaminase act ivity in both cell types revealed a single highly cross-reactive 72-kD band when screened with antibodies to guinea pig tissue transglutaminase. Furth ermore, the proteins immunoprecipitated by this antiserum in vivo exhibited transglutaminase activity. We propose that this transglutaminase is respon sible for an early cell wall protein cross-linking event that temporally pr ecedes the oxidative cross-linking mediated by extracellular peroxidases.