Intercellular communication-filling in the gaps.

Coordination and synchrony of a variety of cellular activities in tissues of plants and animals occur as a consequence of the transfer of low molecular weight biosynthetic and signaling molecules through specialized structures (plasmodesmata in plant cells and gap junctions in mammalian cells) that form aqueous channels between contacting cells. Investigations with rat liver demonstrated that cell-cell communication is mediated by a 32 kilodalton polypeptide that forms a hexameric pore structure in the plasma membrane. Following association with the same structure in a contiguous cell, a trans-double membrane channel is created that has been termed a gap junction. In plant tissue, long tubelike structures called plasmodesmata are suggested to serve a similar cell-cell linking function between cytoplasmic compartments. Although morphologically distinct, dynamic observations suggest similarities in transport properties between gap junctions and plasmodesmata. Recent work now provides evidence that these functional similarities may reflect a more profound identity between the paradigm animal gap junction polypeptide (32 kilodalton rat liver polypeptide) and an immunologically homologous protein localized to plant plasma membrane/cell wall fractions that may be a component of plasmodesmata.