title: Synthetic fragments of the CD4 receptor cytoplasmic domain and large polycations alter the activities of the pp56lck tyrosine protein kinase.
abstract: In CD4+ T cells, the src-like tyrosine kinase pp56lck is associated with the CD4 receptor and cross-linking of CD4 results in the activation of this enzyme. The mechanism responsible for this activation is not known, although there is evidence that the activities of the src family of enzymes are regulated by tyrosine phosphorylation. Here we report that pp56lck-catalyzed angiotensin II phosphorylations are activated 20-fold in vitro by synthetic peptides reproducing portions of the murine CD4 cytoplasmic domain. This activation is described by a dissociation constant of about 2 microM. The pp56lck-catalyzed phosphorylation of other peptide substrates are effected less and in one case not at all by the peptide modulators, indicating that these CD4 sequences alter the substrate specificity of pp56lck. In contrast, peptides reproducing sequences from the CD8 receptor have a charge and size similar to the CD4 peptides, yet are vastly less effective at modulating pp56lck activities. High ionic strengths inhibit the CD4 peptide-induced modulation of pp56lck phosphotransferase activities, suggesting that charge-charge interactions are important for this process. In addition, the modulation of pp56lck activities by peptides reproducing the CD4 cytoplasmic domain are reproduced by polycations significantly larger than the CD4 cytoplasmic domain but not by those of similar size. The modulations both by CD4 peptides and the polycations do not depend on enzyme tyrosine phosphorylations.
authors.affiliation: Department of Biochemistry, University of Rochester Medical Center, New York 14642.
authors.forename: H N
authors.initials: HN
authors.lastname: Bramson
authors.forename: J E
authors.initials: JE
authors.lastname: Casnellie
authors.forename: H
authors.initials: H
authors.lastname: Nachod
authors.forename: L M
authors.initials: LM
authors.lastname: Regan
authors.forename: C
authors.initials: C
authors.lastname: Sommers
mesh_terms: D000595:Amino Acid Sequence; D000804:Angiotensin II / Q000378:metabolism; D000945:Antigens, Differentiation, T-Lymphocyte / Q000138:chemical synthesis / Q000378:metabolism; D015704:CD4 Antigens / Q000138:chemical synthesis / Q000378:metabolism*; D016827:CD8 Antigens; D002384:Catalysis; D002412:Cations; D003593:Cytoplasm / Q000378:metabolism; D004789:Enzyme Activation; D007700:Kinetics; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck); D008969:Molecular Sequence Data; D009994:Osmolar Concentration; D010449:Peptide Mapping; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D013379:Substrate Specificity; D014357:Trypsin
chemical_list: D000945:Antigens, Differentiation, T-Lymphocyte; D015704:CD4 Antigens; D016827:CD8 Antigens; D002412:Cations; D000804:Angiotensin II; D011505:Protein-Tyrosine Kinases; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck); D014357:Trypsin