Nucleolin: A multifunctional major nucleolar phosphoprotein

Nucleolin is a major protein of exponentially growing eukaryotic cells wher e it is present in abundance at the heart of the nucleolus. It is highly co nserved during evolution. Nucleolin contains a specific bipartite nuclear l ocalization signal sequence and possesses a number of unusual structural fe atures. It has unique tripartite structure and each domain performs a speci fic function by interacting with DNA or RNA or proteins. Nucleolin exhibits intrinsic self-cleaving, DNA helicase, RNA helicase and DNA-dependent ATPa se activities. Nucleolin also acts as a sequence-specific RNA binding prote in, an autoantigen, and as the component of a B cell specific transcription factor. Its phosphorylation by cdc2, CK2, and PKC-zeta modulate some of it s activities. This multifunctional protein has been implicated to be involv ed directly or indirectly in many metabolic processes such as ribosome biog enesis (which includes rDNA transcription, pre-rRNA synthesis, rRNA process ing, ribosomal assembly and maturation), cytokinesis, nucleogenesis, cell p roliferation and growth, cytoplasmic-nucleolar transport of ribosomal compo nents, transcriptional repression, replication, signal transduction, induci ng chromatin decondensation and many more (see text). In plants it is devel opmentally, cell-cycle, and light regulated. The regulation of all these fu nctions of a single protein seems to be a challenging puzzle.