Phosphopeptide patterns of the ribosomal protein S6 following stimulation of guineapig parotid glands by secretagogues involving either cAMP or calcium as second messenger

AbstractStimulation of secretion in exocrine cells is associated with the incorporation of up to 3 to 4 phosphates into the ribosomal protein S6. This occurs with secretagogues involving either cAMP or free calcium as second messenger. An analysis of the phosphorylation pattern of S6 from stimulated guineapig parotid glands reveals 3 phosphopeptides (termed A,B,C). The phosphopeptide pattern was identical for cAMP- or calcium-mediated stimulation, whereas phosphorylation of the S6 protein in vitro with catalytic subunit of cAMP-dependent protein kinase resulted only in the formation of phosphopeptides A and C. Therefore, secretagogue-mediated phosphorylation is not or not exclusively catalyzed by cAMP-dependent protein kinase even when cAMP is the second messenger

The protein disulphide-isomerase family: unravelling a string of folds.

The mammalian protein disulphide-isomerase (PDI) family encompasses several highly divergent proteins that are involved in the processing and maturation of secretory proteins in the endoplasmic reticulum. These proteins are characterized by the presence of one or more domains of roughly 95-110 amino acids related to the cytoplasmic protein thioredoxin. All but the PDI-D subfamily are composed entirely of repeats of such domains, with at least one domain containing and one domain lacking a redox-active -Cys-Xaa-Xaa-Cys- tetrapeptide. In addition to their known roles as redox catalysts and isomerases, the last few years have revealed additional functions of the PDI proteins, including peptide binding, cell adhesion and perhaps chaperone activities. Attention is now turning to the non-redox-active domains of the PDIs, which may play an important role in all of the known activities of these proteins. Thus the presence of both redox-active and -inactive domains within these proteins portends a complexity of functions differentially accommodated by the various family members