Isolation of a germin-like protein with manganese superoxide dismutase activity from cells of a moss, Barbula unguiculata

A novel extracellular Mn-superoxide dismutase (SOD) was isolated from a mos s, Barbula unguiculata. The SOD was a glycoprotein; the apparent molecular mass of its native form was 120 kDa, as estimated by gel filtration chromat ography, and that of its monomer was 22,072 Da, as estimated by time of fli ght mass spectroscopy. The protein had manganese with a stoichiometry of 0. 80 Mn/monomer. The cDNA clone for a gene encoding the extracellular Mn-SOD was isolated. Sequence analysis showed that it has a strong similarity to g ermin (oxalate oxidase) and germin-like proteins (GLPs) of several plant sp ecies and possesses all the characteristic features of members of the germi n family. The clone encoding this extracellular Mn-SOD was therefore design ated B. unguiculata GLP (BuGLP). BuGLP had no oxalate oxidase activity. In addition, the cDNA for a gene encoding the moss mitochondrial Mn-SOD was is olated. Its amino acid sequence had little similarity to that of BuGLP, eve n though a close similarity was observed among the mitochondrial Mn-SODs of various organisms. BuGLP was the first germin-like protein that was really demonstrated to be a metalloprotein with Mn-SOD activity but no oxalate ox idase activity.