Dephosphorylation of one 40S ribosomal protein in MPC 11 cells induced by hypertonic medium

Abstract

Summary Mouse myeloma cells (MPC 11) were grown at a high cell density in the presence of [32P] orthophosphate. The tonicity of the medium was raised by 100 mM NaCl in half of the suspension culture after 4 hrs of incubation. The proportions of messenger-free ribosomes and polysomes from salt-treated and control cells were quantitated after separation by sucrose gradient centrifugation and the four ribosomal preparations were analysed by two-dimensional gel electrophoresis. Two ribosomal phosphoproteins, S3 and L28 (nomenclature of Martini and Gould; ref. 1) were detected by autoradiography in all four ribosomal protein patterns. Quantitation of 32P-incorporation revealed that a positive correlation exists between the degree of phosphorylation of S3, and the amount of ribosomes engaged in protein synthesis. S3 was highly phosphorylated in polysomes of control cells, only slightly phosphorylated in polysomes of salt-treated cells and almost completey dephosphorylated in messenger-free ribosomes of salt-treated cells. The 32Pi-incorporation of L28 was much less effected by the hypertonic medium.