AJP - Cell Physiology, Vol 258, Issue 5 C889-C901, Copyright © 1990 by American Physiological Society
Identification of a 50-kDa Ca-, cAMP-, and cGMP-dependent epithelial phosphoprotein as a cAMP regulatory protein
C. Toskulkao and M. C. Rao
Department of Physiology and Biophysics, University of Illinois, College of Medicine, Chicago 60680.
Multiple second messengers, presumably acting via protein phosphorylation
mechanisms, regulate flounder intestinal ion transport. We recently
reported [C. Toskulkao, N. T. Nash, K. Leach, and M. C. Rao. Am. J.
Physiol. 258 (Cell Physiol. 27): C879-C888, 1990] that this tissue
possesses adenosine 3',5'-cyclic monophosphate (cAMP)- and guanosine
3',5'-cyclic monophosphate (cGMP)-specific protein kinases, types II and
III Ca-calmodulin kinases, and very low levels of protein kinase C. These
results correlate with ion transport studies in which cGMP and Ca were
shown to inhibit salt absorption, cAMP to increase anion permeability, and
phorbol esters to have no effect. In the present study we characterized in
detail a 50-kDa protein the phosphorylation of which is regulated by more
than one second messenger. The 50-kDa (pI 5.2) phosphoprotein is present in
both the cytosol (50 kDa-C) and particulate (50 kDa-P) fractions and
appears to be regulated by Ca, cAMP, and cGMP. Although the pI and Mr of
the regulated proteins are identical, there are differences in the
regulation of 50 kDa-P and 50 kDa-C. The phosphorylation of 50 kDa-P is
high in the basal state, and Ca and cGMP enhance this. cAMP has a biphasic
effect, increasing it at low and decreasing it at high protein
concentrations. The isoquinoline derivatives H-8 [50% effective dose (ED50)
approximately 2.3 microM] and H-7 (ED50 approximately 45 microM) inhibit
basal 50 kDa-P phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)
