Maturation of eukaryotic ribosomes: acquisition of functionality

Abstract

In eukaryotic cells, ribosomes are pre-assembled in the nucleus and exported to the cytoplasm where they undergo final maturation. This involves the release of trans-acting shuttling factors, transport factors, incorporation of the remaining ribosomal proteins, and final rRNA processing steps. Recent work, particularly on the large (60S) ribosomal subunit, has confirmed that the 60S subunit is exported from the nucleus in a functionally inactive state. Its arrival in the cytoplasm triggers events that render it translationally competent. Here we focus on these cytoplasmic maturation events and speculate why eukaryotic cells have evolved such an elaborate maturation pathway.

Figure 1. Cytoplasmic maturation events in the 60S biogenesis pathway

(a) Summary of the shuttling trans-acting factors (Rlp24, Mrt4, Tif6, Alb1) and transport factors (Nmd3, Arx1) that are present on pre-60S particles as they arrive in the cytoplasm. Black bars indicate that these pre-60S factors block the association of factors important for function or maturation of the 60S subunit, including Rpl24, the stalk, composed of the P-proteins and the 40S subunit. Until the factors are removed, the pre-60S is not translationally active. (b) Summarizes the cytoplasmic factors (Drg1, Rei1, Sdo1, Efl1, Lsg1 and Yvh1) that are required for the release of the depicted shuttling trans-acting factors and transport factors.