Transport of Balbiani ring granules through nuclear pores in Chironomus tentans.

Balbiani ring granules are premessenger RNP particles synthesized in the larval salivary glands of the dipteran Chironomus tentans and containing the genetic information for large-sized secretory proteins. The granules in the nucleoplasm consist of a thin elementary RNP fiber tightly packed into an RNP ribbon, which is bent into a ring-like shape. When the 50-nm granule is translocated through the nuclear pore, it attains an elongated conformation. We have now demonstrated that the particle in transition can be described as an RNP ribbon with the same thickness and the same minimal width as the ribbon of the nucleoplasmic granules. The ribbon in the pore is, however, somewhat longer and it lacks the broad regions of the ribbon in the nucleoplasmic granule. We conclude that when entering the pore the bent ribbon of the Balbiani ring granule is being straightened and also somewhat drawn out, while its width is accommodated to the maximal size of the pore, i.e., about 25 nm. On the cytoplasmic side the ribbon is unpacked, and the elementary fiber can be seen extending into cytoplasm, probably being available for polysome assembly.