Histochemistry and Cell Biology

, Volume 121, Issue 4, pp 343–350

Juxtamembrane localization of the protein phosphatase-1 inhibitor protein PHI-1 in smooth muscle cells


  • Nikolaos A. Tountas
    • Center for Cell SignalingUniversity of Virginia School of Medicine
  • James W. Mandell
    • Department of Pathology (Neuropathology)University of Virginia Health System
  • Allen D. Everett
    • Department of Pediatrics and the Cardiovascular Research CenterUniversity of Virginia Health System
    • Department of PediatricsJohns Hopkins School of Medicine
    • Center for Cell SignalingUniversity of Virginia School of Medicine
Original Paper

DOI: 10.1007/s00418-004-0642-8

Cite this article as:
Tountas, N.A., Mandell, J.W., Everett, A.D. et al. Histochem Cell Biol (2004) 121: 343. doi:10.1007/s00418-004-0642-8


Protein phosphorylation regulates many fundamental processes and protein phosphatase-1 (PP1) is a major phosphatase that determines the levels of Ser/Thr phosphorylation. Regulatory subunits and inhibitor phosphoproteins control PP1 activity. PHI-1 is a member of a family of PP1 inhibitor phosphoproteins that was discovered based on sequence similarity to the known inhibitor CPI-17. To learn more about PHI-1 we determined the tissue distribution of PHI-1 in embryonic and adult tissues, and examined its cellular localization by immunohistochemistry. In the embryo PHI-1 appeared first in the heart at E10, and by E15 it was detected in multiple tissues. Expression in adult tissues was strikingly different, with PHI-1 detected primarily in smooth muscles in the intestine, blood vessels, and male and female genitourinary tracts. PHI-1 also was highly expressed in the endothelial layer of blood vessels. Both PHI-1 and CPI-17 are expressed predominantly in adult smooth muscles. Whereas CPI-17 staining was diffuse PHI-1 was concentrated along the cell membrane in distinct foci, detected by confocal and electron microscopy. The common tissue distribution but different cellular localization of PHI-1 and CPI-17 suggest distinctive physiological roles for these two PP1 inhibitors.



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© Springer-Verlag 2004