A kinase that transfers the gamma-phosphoryl group of GTP to proteins of eukaryotic 40S ribosomal subunits.

An enzyme in rat-liver cytosol transferred the gamma-phosphoryl of GTP to serine and threonine residues of at least four proteins (S6, S10, S14 or S15, and S17) of the small (40S) subunit of rat-liver ribosomes. A number of nonribosomal proteins in the enzyme preparation were also phosphorylated; they were preferentially and tightly bound to the large subunit. The enzyme could be distinguished from protein kinase-ATP (which also phosphorylated ribosomal proteins) by a number of criteria: (1) GTP was the phosphoryl donor; (2) the pattern of phosphorylation of ribosomal proteins by the two enzymes was different; and (3) the protein kinase that used GTP as the phosphoryl donor was not stimulated by cyclic AMP (or by cyclic GMP).