Diffusive and nondiffusive proteins in vivo.

Proteins within the living cell potentially possess different spatial localizations and a wide range of diffusivities. At one extreme, proteins may be essentially nondiffusive-that is, highly localized structural components of the cytoplasm and/ or nucleus, with very long or infinite residence-times. An extensive, proteinaceous cytomatrix network consisting ofthe polymerized proteins of microtubules, microfilaments, and intermediate filaments, as well as a host of tubule- and filament-associated proteins that apparently serve as structural interconnectors and functional modulators ofthe cytomatrix, does pervade the cell (1). Among the recognized cytomatrix proteins, intermediate filament proteins appear to be archetypical nondiffusive proteins. At the other extreme, some proteins may diffuse freely, with essentially zero residence-times, within the aqueous