ISSN:
1573-4919
Keywords:
ribosomal protein
;
hepatectomy
;
cycloheximide
;
insulin
;
phosphopeptide mapping
;
phosphoprotein phosphatases
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
,
Medicine
Notes:
Summary Spontaneous S6 phosphatase activities dephosphorylating Ser(P)-235 and Ser(P)-236 of the ribosomal protein S6 were measured and compared in microsomes and cytosol of rat liver. The substrate used, small (40S) ribosomal subunits 32P-labelled in vitro by protein kinase A, contained phosphorylated S6 (mainly in the dephosphorylated form) and some minor phosphorylated species. The microsomal and cytosolic S6 phosphatase activities displayed a number of distinct properties. The microsomal activity, representing ca 20% of the S6 phosphatase activity in the post-mitochondrial supernatant, was mainly due to a type-1 phosphatase and dephosphorylated only S6. The remaining post-mitochondrial S6 phosphatase activity, which was fully recovered in the cytosol, and appeared to result from a combination of type-1 (43%) and type 2 (57%) phosphatases, acted on S6 as well as on the minor phosphorylated species. The microsomal activity was 50% inhibited by MgCl2 (l0 mM) and was stimulated at least 4.3 fold by MnCl2 (1 mM), while the cytosolic activity was inhibited only 18% by Mg2+ (10 mM) and was increased 2.2 fold by Mn2+ (1 mM). The microsomal activity was increased 10% (P 〈 0.06) by lower doses of insulin (25 U/Kg) and 14% (P 〈 0.05) by vanadate, but was not significantly (P 〉 0.10) affected by larger doses of insulin (100 U/kg), hepatectomy or cycloheximide. By comparison the cytosolic S6 phosphatase activity was unresponsive to insulin and vanadate, but was decreased 14% and 17% (P 〈 0.05) by hepatectomy and cycloheximide. It is concluded that (i) there. are clear differences between the microsomal and cytosolic S6 phosphatase activities, which may be relevant to their specific functions in the cell, and (ii) the inhibition of cytosolic S6 phosphatase activity by hepatectomy and cycloheximide may contribute to the increase in hepatic S6 phosphorylation induced by these treatments.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00225513
