Ca2+-independent and Ca2+/GTP-binding protein-controlled exocytosis in a plant cell

Exocytosis allows the release of secretory products and the delivery of new membrane material to the plasma membrane. So far, little is known about the underlying molecular mechanism and its control in plant cells. We have used the whole-cell patch-clamp technique to monitor changes in membrane capacitance to study exocytosis in barley aleurone protoplasts. To investigate the involvement of Ca2+ and GTP-binding proteins in exocytosis, protoplasts were dialyzed with very low (<2 nM) and high (1 mu M) free Ca2+ and nonhydrolyzable guanine nucleotides guanosine 5'-gamma-thio]triphosphate (GTP[gamma S]) or guanosine 5'-[beta-thio]diphosphate (GDP[beta S]). With less than 2 nM cytoplasmic free Ca2+, the membrane capacitance increased significantly over 20 min. This increase was not altered by GTP[gamma S] or GDP[beta S]. In contrast, dialyzing protoplasts with 1 mu M free Ca2+ resulted in a large increase in membrane capacitance that was slightly reduced by GTP[gamma S] and strongly inhibited by GDP[beta S]. We conclude that two exocytotic pathways exist in barley aleurone protoplasts: one that is Ca2+-independent and whose regulation is currently not known and another that is stimulated by Ca2+ and modulated by GTP-binding proteins. We suggest that Ca2+-independent exocytosis may be involved in cell expansion in developing protoplasts. Ca2+-stimulated exocytosis may play a role in gibberellic acid-stimulated alpha-amylase secretion in barley aleurone and, more generally, may be involved in membrane resealing in response to cell damage.