Sorting of proteins to vacuoles in plant cells

Abstract

An individual plant cell may contain at least two functionally and structurally distinct types of vacuoles: protein storage vacuoles and lytic vacuoles. Presumably a cell that stores proteins in vacuoles must maintain these separate compartments to prevent exposure of the storage proteins to an acidified environment with active hydrolytic enzymes where they would be degraded. Thus, the organization of the secretory pathway in plant cells, which includes the vacuoles, has a fascinating complexity not anticipated from the extensive genetic and biochemical studies of the secretory pathway in yeast. Plant cells must generate the membranes to form two separate types of tonoplast, maintain them as separate organelles, and direct soluble proteins from the secretory flow specifically to one or the other via separate vesicular pathways. Individual soluble and membrane proteins must be recognized and sorted into one or the other pathway by distinct, specific mechanisms. Here we review the emerging picture of how separate plant vacuoles are organized structurally and how proteins are recognized and sorted to each type.

Isolation and Properties of Floral Defensins from Ornamental Tobacco and Petunia

“…This indicates that a role in vacuolar targeting should be considered for the C-terminal prodomain on NaD1, although this needs to be confirmed experimentally because there are no consensus sequences that define C-terminal vacuolar-sorting determinants (Neuhaus and Rogers, 1998). However, the C-terminal prodomain of NaD1 is rich in acidic and hydrophobic amino acids, a common feature of C-terminal vacuolar-sorting determinants (Nielson et al, 1996).…”

Identification and Characterization of a Prevacuolar Compartment in Stigmas of Nicotiana alata

“…Proteins destined for the vacuole enter the secretory pathway via the endoplasmic reticulum (ER) and are diverted from the default pathway of secretion by specific sorting signals that redirect them to the vacuole. Three classes of sorting signals have been identified: N-terminal propeptide domains that share a conserved NPIR motif, C-terminal propeptide (CTPP) domains that share an amphipathic helical structure but are not conserved in sequence, and regions of mature proteins that probably are exposed on surface loops (reviewed in Neuhaus and Rogers, 1998).…”

Tansley Review No. 108

“…For plants, evidence from kinetic analyses and biochemical studies outlined here suggests that at least two different pools of vesicles are available for delivery to the plasma membrane alone. These data, and the evidence for specialization between vacuole compartments in plant cells (Beevers & Raikhel, 1998 ;Neuhaus & Rogers, 1998) implies the situation in plants is likely to prove even more complex. Characterizing these pathways, their functions and controls therefore will be a major challenge for the future.…”

“…These new technologies have already begun to yield details wholly unexpected from past studies (Thiel & Battey, 1998). Various aspects of protein sorting and secretion in plant cells have been discussed broadly in a number of excellent reviews (Battey & Blackbourn, 1993 ;Beevers & Raikhel, 1998 ;Neuhaus & Rogers, 1998 ;Robinson et al, 1998 ;Thiel & Battey, 1998 ;Battey et al, 1999 ;Sanderfoot & Raikhel, 1999). Here we focus on recent findings relating to the mechanisms of vesicle trafficking and the background to these developments, highlighting both current understanding of the molecular events of secretion and the gaps therein, as well as discussing emerging themes from work with plants.…”

“…Additionally, plant cells appear to maintain several distinct vacuolar compartments with different functions. Exocytosis and recycling through these compartments thus adds a further layer of complexity to vesicle trafficking in plants (Neuhaus & Rogers, 1998). Each of these pathways must be under tight metabolic control.…”