Functional modules in ribosomal protein L5 for ribonucleoprotein complex formation and nucleocytoplasmic transport

Ribosomal protein L5 forms a small, extraribosomal complex with 5 S ribosom al RNA, referred to as the 5 S ribonucleoprotein complex, which shuttles be tween nucleus and cytoplasm in Xenopus oocytes, Mapping elements in L5 that mediate nuclear protein import defines three separate such activities (L5- nuclear localization sequence (NLS)-1, -2, and -3), which are functional in both oocytes and somatic cells, RNA binding activity involves N-terminal a s well as C-terminal elements of L5, In contrast to the full length protein , none of the individual NLSs carrying L5 fragments are able to allow for t he predominating accumulation in the nucleoli that is observed with the ful l-length protein. The separate L5-NLSs differ in respect to two activities. Firstly, only L5-NLS-1 and -3, not L5-NLS-2, are capable of promoting the nuclear transfer of a heterologous, covalently attached ribonucleoprotein c omplex. Secondly, only L5-NLS-1 is able to bind strongly to a variety of di fferent import receptors; those that recognize L5-NLS-2 and -3 have yet to be identified.