A nucleolar protein (green) is first (top) found in splicing speckles (blue) before later moving to nucleolar structures (red).

Protein trafficking between the cytoplasm and the nucleus has been studied extensively; but how do proteins move from one site to another within the nucleus? On page 615, Leung and Lamond focus on the intranuclear trafficking of the RNA-binding protein NHPX, and describe an elegant series of experiments that demonstrates the existence of multiple intranuclear accumulation pathways. The study provides definitive evidence of protein sorting within the nucleus.Although previous studies have suggested that proteins might follow directed paths between intranuclear bodies, the new work provides a detailed analysis of this phenomenon. NHPX localizes primarily to nucleoli, but is capable of binding to both small nucleolar RNAs (snoRNAs) and the spliceosomal U4 small nuclear RNA (snRNA). The authors followed fluorescent fusion proteins to determine that newly expressed NHPX transiently visits splicing speckles in the nucleus before accumulating stably in the nucleolus, a pattern confirmed in multiple cell lines. The move from speckles to nucleolus is apparently unidirectional, and requires new mRNA transcription, suggesting that other factors must be expressed to allow NHPX to leave the speckles.

Leung and Lamond also compare the trafficking of NHPX to that of the SmB protein. In contrast to NHPX, SmB accumulates first in Cajal bodies and in the nucleolus, before finally accumulating in speckles, indicating that the two proteins move through distinct sorting pathways with in the nucleus.

The unidirectional movement of NHPX, combined with its previously observed binding properties, suggests that NHPX might interact with U4 snRNA in speckles, possibly facilitating the maturation of nuclear proteins or RNP complexes before moving to the nucleolus. The authors are now trying to determine whether other nuclear proteins share the separate trafficking routes used by NHPX and SmB, and they hope to identify the molecular mechanisms responsible for these pathways. ▪