THE ACCESSIBILITY OF YEAST RIBOSOMAL-PROTEIN L1 AS PROBED BY PROTEOLYSIS AND SITE-DIRECTED MUTAGENESIS IS DIFFERENT IN INTACT 60-S AND 80-SRIBOSOME

Accessible regions of protein L1 in intact 60 and 80 S ribosomes from Saccharomyces cervisiae were first detected by controlled proteolysis. The N-terminal region of L1 in either 60 S or 80 S particles, was ina ccessible to proteases, but the central and C-terminal regions were ac cessible. The accessibility of the central region differed depending o n the ribosome state. These regions were further examined by determina tion of the chemical reactivity of specific cysteine residues introduc ed into these regions by site-directed mutagenesis. All cysteine mutan t proteins were capable of binding yeast 5 S rRNA in vitro and the rib osomes containing the mutant proteins were functional in vivo. Residue s Cys-257 and Cys-275 were modified in both the 60 and 80 S ribosomes but the modification rates were different in the two ribosome states. Both residues Cys-62 and Cys-286 were inaccessible in 80 S or 60 S rib osomes. Taken together, the present study identified several accessibl e regions of L1 in intact ribosomes and further showed that the access ibility of some of the regions was altered upon ribosomal subunit asso ciation. The most likely interpretation of these results is that the c onformation of the ribosomal protein L1 was altered upon ribosomal sub unit association.