Diane C. Bassham

Unique features of the plant vacuolar sorting machinery

Multiple types of vacuoles can exist within the same plant cell, and different vesicle-trafficking pathways transport proteins to each of them. Recent work has identified proteins unique to each vacuole type, and the transport pathways have begun to be elucidated. Plant trafficking proteins are usually encoded by small gene families, the different members of which have distinct functions in the endomembrane system.

Transport of proteins in eukaryotic cells: more questions ahead

Some newly synthesized proteins contain signals that direct their transport to their final location within or outside of the cell. Targeting signals are recognized by specific protein receptors located either in the cytoplasm or in the membrane of the target organelle. Specific membrane protein complexes are involved in insertion and translocation of polypeptides across the membranes. Often, additional targeting signals are required for a polypeptide to be further transported to its site of function. In this review, we will describe the trafficking of proteins to various cellular organelles (nucleus, chloroplasts, mitochondria, peroxisomes) with emphasis on transport to and through the secretory pathway.

Transport proteins in the plasma membrane and the secretory system

The organelles of the plant secretory system and the plasma membrane each contain a specific complement of resident integral membrane proteins, which provide each organelle with some of its unique characteristics. Over the past few years, genes encoding some of these proteins have been isolated and this has allowed the function and localization of the encoded proteins to be determined. A number of the genes encode proteins involved in transport processes, both in the trafficking of proteins between membranes and the transport of solutes across membranes. However, little is known about the majority of membrane proteins within the endomembrane system. A more complete understanding of the processes occurring within this system awaits the identification and analysis of many more of its components and the interactions between them. This will also allow important questions to be addressed regarding the mechanisms by which membrane proteins are correctly localized and assembled in the secretory pathway.