MrpL36p, a highly diverged L31 ribosomal protein homolog with additional functional domains in Saccharomyces cerevisiae mitochondria

Translation in mitochondria utilizes a large complement of ribosomal proteins. Many mitochondrial ribosomal components are clearly homologous to eubacterial ribosomal proteins, but others appear unique to the mitochondrial system. A handful of mitochondrial ribosomal proteins appear to be eubacterial in origin but to have evolved additional functional domains. MrpL36p is an essential mitochondrial ribosomal large-subunit component in Saccharomyces cerevisiae. Increased dosage of MRPL36 also has been shown to suppress certain types of translation defects encoded within the mitochondrial COX2 mRNA. A central domain of MrpL36p that is similar to eubacterial ribosomal large-subunit protein L31 is sufficient for general mitochondrial translation but not suppression, and proteins bearing this domain sediment with the ribosomal large subunit in sucrose gradients. In contrast, proteins lacking the L31 domain, but retaining a novel N-terminal sequence and a C-terminal sequence with weak similarity to the Escherichia coli signal recognition particle component Ffh, are sufficient for dosage suppression and do not sediment with the large subunit of the ribosome. Interestingly, the activity of MrpL36p as a dosage suppressor exhibits gene and allele specificity. We propose that MrpL36p represents a highly diverged L31 homolog with derived domains functioning in mRNA selection in yeast mitochondria.