The distribution of membrane-bound 14-3-3 proteins in organelle-enriched fractions of germinating lily pollen

Proteins of the 14-3-3 family show a broad range of activities in plants, depending on their localisation in different cellular compartments. Different organelle membranes of pollen grains and pollen tubes of Lilium longiflorum Thunb. were separated simultaneously using optimised discontinuous sucrose density centrifugation. The obtained organelle-enriched fractions were identified as vacuolar, Golgi, endoplasmic reticulum and plasma membranes, according to their marker enzyme activities, and were assayed for membrane-bound 14-3-3 proteins by immunodetection. 14-3-3 proteins were detected in the cytoplasm as well as in all obtained organelle fractions but were also released into the extracellular medium. In pollen grains, much more plasma membrane-bound 14-3-3 proteins were detected than in the PM-enriched fraction of pollen tubes, whereas the level of Golgi- and ER-associated 14-3-3 proteins was similar in pollen grains and tubes. This shift in the localisation of membrane-associated 14-3-3 proteins is probably correlated with a change in the major function of 14-3-3 proteins, e.g., perhaps changing from initiating pollen grain germination by activation of the PM H +-ATPase to recruitment of membrane proteins via the secretory pathway during tube elongation.