A soluble mono(ADP-ribosyl)transferase was detected in dormant spores of Soluble proteins incubated with [P]NAD revealed, after two-dimensional electrophoresis, three major ADP-ribosylated substrates with molecular masses of 38, 37 and 36 kDa and pl values of 6.9, 8.1 and 4.6, respectively. When these endogenous substrates were first ADP-ribosylated with [P]NAD and then incubated for different times with either 3 M hydroxylamine (pH 7.0) or 1 mM HgCI, only hydroxylamine released the incorporated radioactivity after 30 min incubation. Additionally, agmatine was used as a substrate for this enzyme. These data suggest that the mono(ADP-ribosyl)transferase is an arginine-specific enzyme. This enzymic activity was stimulated by 10 mM MgCI and by 250 µM of the nitric-oxide-releasing agent sodium nitroprusside, and inhibited by 8 mM benzamide, 0.4 mM -iodobenzylguanidine and 0.5 mM novobiocin. The three ADP-ribosylation inhibitors affected the germination of spores, leaving them as swollen cells. The effect of MgCI, GTP and ATP on the ADP-ribosylation of the endogenous proteins was studied. The presence of two additional [P]ADP-ribosylated proteins of 57 and 55 kDa was observed in the absence of MgCI. An increase in incorporation of radioactivity into the 55 kDa band was observed when the assay was performed in the presence of GTP or ATP. The addition of Mg together with either or both nucleotides eliminated the appearance of the 57 and 55 kDa bands, but intensified the 37 kDa band. Photoaffinity-labelling of the soluble fraction with [α-P]GTP revealed a 55 kDa band together with other proteins of 32 and 17 kDa. These results suggest that among the five different endogenous substrates for the fungal mono(ADP-ribosyl)transferase, the 55 kDa protein may be a GTP-binding protein.


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