Naturally occurring proteolytic antibodies: selective immunoglobulin M-catalyzed hydrolysis of HIV gp120.
We report the selective catalytic cleavage of the HIV coat protein gp120, a B cell superantigen, by IgM antibodies (Abs) from uninfected humans and mice that had not been previously exposed to gp120. The rate of IgM-catalyzed gp120 cleavage was greater than of other polypeptide substrates, including the bacterial superantigen protein A. The kinetic parameters of gp120 cleavage varied over a broad range depending on the source of the IgMs, and turnover numbers as great as 2.1/min were observed, suggesting that different Abs possess distinct gp120 recognition properties. IgG Abs failed to cleave gp120 detectably. The Fab fragment of a monoclonal IgM cleaved gp120, suggesting that the catalytic activity belongs to the antibody combining site. The electrophoretic profile of gp120 incubated with a monoclonal human IgM suggested hydrolysis at several sites. One of the cleavage sites was identified as the Lys(432)-Ala(433) peptide bond, located within the region thought to be the Ab-recognizable superantigenic determinant. A covalently reactive peptide analog (CRA) corresponding to gp120 residues 421-431 with a C-terminal amidino phosphonate diester mimetic of the Lys(432)-Ala(433) bond was employed to probe IgM nucleophilic reactivity. The peptidyl CRA inhibited the IgM-catalyzed cleavage of gp120 and formed covalent IgM adducts at levels exceeding a control hapten CRA devoid of the peptide sequence. These observations suggest that IgMs can selectively cleave gp120 by a nucleophilic mechanism and raise the possibility of their role as defense enzymes.
Chemical Immunology and Therapeutics Research Center, Department of Pathology and Laboratory Medicine, University of Texas-Houston Medical School, Houston, Texas 77030, USA. Sudhir.Paul@uth.tmc.edu
Sudhir
S
Paul
Sangeeta
S
Karle
Stephanie
S
Planque
Hiroaki
H
Taguchi
Maria
M
Salas
Yasuhiro
Y
Nishiyama
Beverly
B
Handy
Robert
R
Hunter
Allen
A
Edmundson
Carl
C
Hanson
D000328:Adult; D000818:Animals; D017151:Antibodies, Catalytic / Q000378:metabolism*; D000911:Antibodies, Monoclonal / Q000378:metabolism; D002384:Catalysis; D005260:Female; D015699:HIV Envelope Protein gp120 / Q000276:immunology* / Q000378:metabolism*; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000378:metabolism; D007075:Immunoglobulin M / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008875:Middle Aged; D008954:Models, Biological
D017151:Antibodies, Catalytic; D000911:Antibodies, Monoclonal; D015699:HIV Envelope Protein gp120; D007140:Immunoglobulin Fab Fragments; D007075:Immunoglobulin M