Processive versus distributive methyl group transfer was assessed for pea 
Rubisco large subunit methyltransferase, a SET domain protein lysine 
methyltransferase catalyzing the formation of trimethyllysine-14 in the large 
subunit of Rubisco. Catalytically competent complexes between an immobilized 
form of des(methyl) Rubisco and Rubisco large subunit methyltransferase were 
used to demonstrate enzyme release that was co-incident with and dependent on 
formation of trimethyllysine. Catalytic rate constants determined for formation 
of trimethyllysine were considerably lower ( approximately 10-fold) than rate 
constants determined for total radiolabel incorporation from 
[3H-methyl]-S-adenosylmethionine. Double-reciprocal velocity plots under 
catalytic conditions favoring monomethyllysine indicated a random or ordered 
reaction mechanism, while conditions favoring trimethyllysine suggested a hybrid 
ping-pong mechanism. These results were compared with double-reciprocal velocity 
plots and product analyses obtained for HsSET7/9 (a monomethyltransferase) and 
SpCLR4 (a dimethyltransferase) and suggest a predictive ability of 
double-reciprocal velocity plots for single versus multiple methyl group 
transfers by SET domain protein lysine methyltransferases. A model is proposed 
for SET domain protein lysine methyltransferases in which initial binding of 
polypeptide substrate and S-adenosylmethionine is random, with polypeptide 
binding followed by deprotonation of the epsilon-amine of the target lysyl 
residue and subsequent methylation. Following methyl group transfer, 
S-adenosylhomocysteine and monomethylated polypeptide dissociate from 
monomethyltransferases, but di- and trimethyltransferases begin a successive and 
catalytically obligatory deprotonation of enzyme-bound methylated lysyl 
intermediates, which along with binding and release of S-adenosylmethionine and 
S-adenosylhomocysteine is manifested as a hybrid ping-pong-like reaction 
mechanism.