Ribosomal protein L4 is a large ribosomal subunit protein that is structurally conserved in all kingdoms of life. In bacterial species, this protein plays key extra-ribosomal functions such as autogenous regulation of its operon. This protein has also been shown to modulate the activity of the ribosomal tunnel, thus affecting the rate of protein synthesis. Experimental evidence indicates that some of these functions might be conserved in eukaryotes. For example, it was recently shown that L4 plays a key role in human ribosome biogenesis. Furthermore, there are several reports of alteration in regulation of L4 mRNA during tissue regeneration in neurons, interaction of L4 with transcriptional factors like PBX2 in pre-B cell lymphocytes in leukemia patients, and varied regulation of L4 mRNA in prostate cancer cell lines. This indicates that L4 has key extra ribosomal functions along with its role in ribosomal biogenesis and function.

My research shows that in vivo depletion of Rpl4p in Saccharomyces cerevisiae results in severe loss of 60S ribosomal subunits. Our Northern blot, primer extension, and uracil pulse-labeling experiments suggest this might be due to a block in the processing of the 27SA3 precursor RNA into 5.8S and 25S rRNA as well as a delay in processing of 35S precursor. Furthermore depletion of Rpl4p impairs cytoplasmic export of 60S precursor ribosomes from the nucleus. More surprising, cell cycle analysis after depletion of Rpl4p appears to arrest cells in late steps of cytokinesis, resulting in bibudded and tribudded phenotypes unlike the cell cycle defects reported due to depletion of other large subunit proteins and accessory factors involved in ribosome biogenesis. Further experiments are being conducted to examine if the cell cycle defect is direct result of loss of L4 protein or and indirect one.