Sites of phosphorylation and mutation in regulatory subunit of cyclic AMP-dependent protein kinase from S49 mouse lymphoma cells: mapping to structural domains

A novel peptide mapping approach has been used to map sites of charge modification to major structural domains of regulatory subunit (R) of type I cAMP-dependent protein kinase from S49 mouse lymphoma cells. Proteolytic fragments of crude, radiolabeled R were purified by cAMP affinity chromatography and displayed by two-dimensional polyacrylamide gel electrophoresis. [35S]methionine-labeled peptides containing sites of mutation or phosphorylation exhibited charge heterogeneity attributable to the modification. Phosphate-containing fragments were also labeled with [32P]orthophosphate to confirm their phosphorylation. Major fragments from [35S]methionine-labeled S49 cell R corresponded in size to carboxyterminal cAMP-binding fragments reported from proteolysis of purified type I Rs from various mammalian species; additional fragments were also visualized. End-specific markers in Rs from some mutant S49 sublines confirmed that cAMP-binding fragments extended to the carboxyterminus of R. Aminoterminal endpoints of fragments could be deduced, therefore, from peptide molecular weights. Clustering of proteolytic cleavage sites within the "hinge-region" separating aminoterminal and carboxyterminal domains of R permitted high resolution mapping in this region: the endogenous phosphate and a "phenotypically-silent" electrophoretic marker mutation fell within a 2.5-kdalton interval at its aminoterminal end. On the other hand, Ka mutations that increase the apparent constant for activation of kinase by cAMP mapped within the large cAMP-binding region of R. A map of charge density distribution within the hinge-region of R was constructed to facilitate structural comparisons between Rs from S49 cells and from other mammalian sources.