Low molecular mass GTP-binding proteins in subcellular fractions of the pancreas: regulated phosphoryl G proteins.

Low molecular mass guanine nucleotide-binding proteins [small guanosine 5'-triphosphate (GTP)-binding proteins] and phosphoproteins of the pancreatic acinar cell were compared by two-dimensional gel electrophoresis. [35S]GTP alpha S blotting analysis of the total cell protein revealed 20 GTP-binding proteins ranging in molecular mass from 20 to 28 kDa and pI of 4.8-6.4. Analysis of 32P-labeled total cell protein revealed over 300 phosphoproteins. The subcellular distribution of the small GTP-binding proteins was examined: 17 were located in the rough endoplasmic reticulum (RER) fraction, 19 in the smooth microsome fraction, 14 in the zymogen granule membrane fraction, and 11 in the cytosolic fraction, with overlap between fractions. Of the GTP-binding proteins, two were also found to be phosphoproteins, one located on the RER and one on the zymogen granule membrane. The phosphorylation of both small GTP-binding proteins was increased by secretagogue stimulation of the cells but with different time courses. The RER small GTP-binding protein demonstrated a rapid and transient increase in 32P labeling, whereas the granule membrane small GTP-binding protein showed an increase at longer times (30 min). Two of the cytosolic small GTP-binding proteins were also seen in particulate fractions, especially in the zymogen granule membrane fraction, suggesting the possibility of cycling between cytosolic and membrane-associated forms.