Nuclear speckles: a model for nuclear organelles

Abstract

Speckles are dynamic subnuclear structures that contain pre-messenger RNA splicing factors and other proteins that are involved in transcription, 3′- end RNA-processing and reversible protein phosphorylation. The formation of speckles is regulated during the cell-division cycle. Splicing factors cycle continually between speckles and the nucleoplasm. Their size and shape results from the dynamic exchange of factors into and out of speckles. A reversible protein phosphorylation mechanism can regulate the movement of speckle components between speckles and other nuclear structures. It is likely that protein–protein interactions are primarily responsible for the formation and integrity of speckles. Speckles contain little or no DNA and are not principal sites of transcription. Instead, they function as assembly/modification sites that can supply active splicing factors to sites of transcription. We propose a 'regulated-exchange' model to account for the steady-state level of proteins in speckles. This envisages that the concentration of factors that are localized in speckles results from a regulated and cell-type-specific basal exchange rate of speckle components.