This study reports the purification and identification of a novel 28 kDa
phosphoprotein from rat pancreatic acini, previously described as being highly regulated by
calcium mobilizing
secretagogues, which we have designated
calcium-regulated heat-stable protein 28 (CRHSP-28). Internal amino acid sequences of purified
CRHSP-28 were obtained following
trypsin digestion and found to match with >95% identity the predicted amino acid sequence of a novel
cDNA recently identified as being highly expressed in human
breast carcinomas. Verification that this
cDNA codes for human
CRHSP-28 was demonstrated by the ability of antiserum raised against purified rat
CRHSP-28 to recognize the recombinant human
protein when expressed in bacteria. Furthermore, this antibody was found to specifically react with
CRHSP-28 in rat acini following one- and two-dimensional electrophoresis and underwent a marked acidic shift in mobility after
cholecystokinin stimulation, a phenomenon indicative of an increase in its phosphorylation.
CRHSP-28 is predicted to be extremely hydrophilic, is phosphorylated entirely on
serine residues, and bears little homology to any known
proteins. Finally, the distribution of the
CRHSP-28 protein in various rat tissues revealed that although it was present at low levels in almost all tissues, it was most highly expressed in pancreas, followed by the gastric, intestinal, and colonic mucosa. In view of its relative abundance throughout the digestive system and its apparent regulation by
calcium-mobilizing agents, this
protein may provide valuable insight into the mechanism(s) of calcium signaling in these tissues.