PHOSPHOPROTEINS IN PLASMA MEMBRANES FROM RAT LIVER CELLS . 32P INCORPORATION INTO PHOSPHOPROTEINS

Three different sorts of phosphoproteins have been characterized in plasma membranes from rat liver cells: alkali unstable phosphoproteins, containing phosphoseryl and phosphothreonyl groups: acid-unstablephosphoproteins containing phosphohistidine; and acyl phosphate-bound phosphoproteins. Alkali-unstable phosphoproteins from plasma membranes are more abundant than in any other structure (7 μg P/mg N protein as compared with 1.3 μg in microsomes). They are also more rapidly labelled. About 30 min after injecting 32P into animals, they attain a maximum specific radioactivity, which is 4 times greater than that of the total cell phosphoproteins. The maximum specific radioactivity is reached within the same time in acid-unstable phosphoproteins as well as in Pi and ATP, which suggests that a rapid exchange of 32P is occurring between these different compounds. In contrast, after labelling in vivo, acyl phosphate-bound phosphoproteins from plasma membranes are found to be 10-20 times less radioactive than alkali-unstable phosphoproteins, but they are very rapidly labelled in vitro when isolated membranes are incubated with [γ-32P]ATP. The radioactivity is K+ and Na+ dependent. © 1969.