ADP-ribosylation of alpha i3C20 by the S1 subunit and deletion peptides of S1 of pertussis toxin.

Recombinant S1 subunit of PT (rS1) and two carboxyl-terminal deletion peptides, C180 and C204, which comprise the amino-terminal 180 and 204 amino acids of S1, respectively, were analyzed for the ability to ADP-ribosylate alpha i3C20, a synthetic peptide composed of the 20 carboxyl-terminal amino acids of the alpha subunit of the heterotrimeric G protein Gi3. Under linear velocity conditions, C180 ADP-ribosylated alpha i3C20 at a 3-fold higher rate than either C204 or rS1. At variable NAD, rS1, C204, and C180 ADP-ribosylated alpha i3C20 with similar initial velocities which followed Michaelis-Menten kinetics. In contrast, at variable alpha i3C20, rS1, C204, and C180 ADP-ribosylated alpha i3C20 with different initial velocities. At variable alpha i3C20, C204- and rS1-catalyzed ADP-ribosylation followed Michaelis-Menten kinetics, while the velocity curve generated by C180 diverged from Michaelis-Menten kinetics. The rates of initial velocity of C180 did not fit the Lineweaver-Burk equation, but could be transformed into the Hill equation which yielded a Hill coefficient of 2. This predicted that C180 possessed cooperativity between the two substrate binding sites. Other experiments showed that C180 ADP-ribosylated alpha i3C20 at 60% of the rate for the ADP-ribosylation of Gt. These data showed that the entire catalytic mechanism for ADP-ribosylation resides within the first 180 amino acids of S1 and that the carboxyl-terminal 55 residues of S1 allow the ADP-ribosylation of alpha i3C20 to proceed via Michaelis-Menten kinetics. These data along with earlier studies (Krueger & Barbieri, 1993) were also consistent with the presence of two Gt protein binding sites within S1.(ABSTRACT TRUNCATED AT 250 WORDS)