Protein recycling from the Golgi apparatus to the endoplasmic reticulum inplants and its minor contribution to calreticulin retention

Using pulse-chase experiments combined with immunoprecipitation and N-glyca n structural analysis, we showed that the retrieval mechanism of proteins f rom post-endoplasmic reticulum (post-ER) compartments is active in plant ce lls at levels similar to those described previously for animal cells, For i nstance, recycling from the Golgi apparatus back to the ER is sufficient to block the secretion of as much as 90% of an extracellular protein such as the cell wall invertase fused with an HDEL C-terminal tetrapeptide. Likewis e, recycling can sustain fast retrograde transport of Golgi enzymes into th e ER in the presence of brefeldin A, However, on the basis of our data, we propose that this retrieval mechanism in plants has little impact on the ER retention of a soluble ER protein such as calreticulin. Indeed, the latter is retained in the ER without any N-glycan-related evidence for a recyclin g through the Golgi apparatus, Taken together, these results indicate that calreticulin and perhaps other plant reticuloplasmins are possibly largely excluded from vesicles exported from the ER, Instead, they are probably ret ained in the ER by mechanisms that rely primarily on signals other than H/K DEL motifs.